So, I started downloading custom content just this week. But two days ago, the game started doing something weird. I can play just like normal, but certain lots are replaced by this water-like substance. I can click on the 'water' and then it shows that I can fish and swim there. The weird thing - if this wasn't already weird- is that the places where my two adult sims are working, still are like normal, along with some other lots, like the criminal space, the supermarket etc. But the school isn't available, so it says something like 'No current school available' when I click on the career icon of my child sim. The lot where the school used to be, has also become water.
I have already removed all the custom content, even the whole mod folder, but the problem isn't fixed yet. I haven't had this problem before I downloaded any kind of custom content.
I don't play with any expansion packs, just the base game.

Can someone help me out?

yeah, famous feature; look there: http://modthesims.info/wiki.php?tit...Blue_Lots_Issue

ElaineNualla is right. You are experiencing the "blue lot" issue. But it can be fixed if you are patient. You have corrupted files in your CC. The Dashboard can fix them. They have to be packages. Sims3packs can be corrupted as well as packages. You should use Delphy's Multi-installer and convert your s3packs to packages, then Dashboard will display ( in red) the bad files and will fix them if you want.

If you have saved your game after seeing the blue lots issue, I am not sure if you can fix the save. Best thing to do is to save frequently when the game runs fine. Never save if you notice an issue when you enter a saved game. Leave the game and try to fix the issue.

Important: You should scan all pieces of CC that you download with Dashboard before installing them in your game. Prevention is better than cure !!