Last time I was able to get into my town (Veronaville) and play my beloved family was when my two adult sims aged into elders. After they did, I played for a while and then exited and saved. Next day, when I felt like playing them again, I chose the same town (Veronaville) and found it only loads up to the third little bar on the top of the loading screen, and then an error message pops up and says "The application has crashed. The application will now terminate."
I'm confused, as I was able to get into my town perfectly fine before. But once my sims aged into elders, the town just won't load.
I used Mootilda's Neighborhood Corruption Detector, and when I ran my town through the application, I got a message saying my town has "no valid sims". What does this mean? My game was perfectly fine before my active family's two adult sims aged into elders. Very confused.
If anyone has any suggestions/solutions it'd be greatly appreciated.

Does it open in simpe? You might have to restore to a backup.

You could try and do a revert back to a previous version by right clicking on the N003 folder in My Doc, "C:\Users\(your computer name)\Documents\EA Games\The Sims 2\Neighborhoods\N003" and chosing Properties and then click on Previous Version and choose one just before this happened.

I'm using Windows 8.1. Where would I find this "previous version" option?
I opened it in SimPE, and under the Resource Tree, there's only 4 assets -- Catalog description, directory of compressed files, ID number and version information. Compared to when I opened another hood, there's a LOT more in other hoods under Resource Tree. ??
I'm not an expert on this stuff, just a casual gamer -- sorry if I come across as a bit vague.

Yes, the file sounds corrupted. Do you have a back up?

No, I don't.. How could've it gone corrupted, though? No sims died, I didn't delete any sims either. Very confused..

Lots of things can cause corruption. Check your objects.package files for all EPs and SPs

The game will sometimes mass delete because it runs into a problem. It's why long-term players like me keep backups. Have you checked to see if the character files are still there?

What names do the character files go as/where are they located?
If they are, how could I possibly retrieve them and put them into a different hood, if possible?
I really hadn't thought of a backup because I assumed something like this couldn't happen to me.
Also, what exactly does "no valid sims" mean? Is this just another term for corruption, or does it have a specific meaning?

It means, I think, that the Neighbourhood scanner can't find the character files. They should be in

My Documents\EA Games\The Sims 2\Neighborhoods\N003\Characters

Unfortunately you can't transfer the character files to a new neighbourhood without instant corruption and likely borking the new hood completely before you even open it.

The only option with a corrupted hood where you have no back up (and can't go back) is to copy the sims' appearance (appearance only - you'd have to rebuild the rest) with the SimSurgery option in SimPE. Which is probably why Jo asked if you could open the neighbourhood file in SimPE.

Sounds like your game wiped all the data from sims in that hood, meaning there are no sims in it that are playable. That type of thing is normally the last stages of long standing corruption which exploded last play session. Try what marka said, and you go exactly where he said to go. To Documents and the Sims 2 folder and find your hood, right click on it. Corruption can happen to anyone, why would it not to you? Always make backups. If you have a backup and worse comes to worse you either restore to the backup or you start rebuilding by extracting your sims. Yes extracting sims was why I asked if you could open it in simpe. Often playing the backup will go the same way as the real hood anyway.

Lots of non-game possible causes, too. Like small children, power outages, antivirus programs . . . I even broke, yes, actually physically broke, a hard drive once. (Not from dropping it--best guess is there was some small flaw in manufacturing and use eventually cracked it.)

I highly suggest you back up any pictures, documents, etc, because backups are always a good plan whether or not your computer is behaving in unwanted ways, and then pursue your game fixing options. Mootilda's last program, SimRetriever, is here: https://www.modthesims.info/download.php?t=503733 and may be of use to you in extracting sims to recreate.

Thank you, everyone. This has been an excellent way for me to get a good understanding of what's happened to my game. I'll try the SimSurgery option in SimPE. I loved my two sims, they had two kids and one had just aged into a YA, and the other was a cute as heck little toddler (Didn't even know genetics system could produce a baby that cute!) It's a shame that they're gone now, but I'll take this as a lesson to make sure there's a backup for my next game.
Another possible idea that I had that may explain why my game got corrupted is because when one of my sims aged into an elder, and it asked for me to age up other sims in the town, I chose an elder to age up, I believe. (Not 100% sure if they were actually an elder but they had grey hair, so I assume they were.) Could that have made something gone wrong? Otherwise I can't think of anything else that would've caused it. Does it happen randomly sometimes?

You can only extract sims in SimPE if you have a viable hood folder, so did you manage to roll it back like marka said?

I'm on Windows 8.1 and can't seem to find the "previous version" button under Properties.

It's one of the tabs when you select Properties - mind you I'm on Win7 so it might have changed for Win8.

When you right click on the folder properties it should have previous versions on a tab at the top of the window. I am also on Win 7 though.

Did a little research and found that Windows 8/8.1 removed the feature and replaced it with something that doesn't even automatically backup your files unless you turn on the feature. Which I had no idea about and obviously did not do.

Well, that's a bummer but explains your difficulty in spotting it.