I recently formatted my computer and Steam Cloud didn't save everything I did. How do I get my levels back? Thanks.
The obvious answer would be to play them again but I assume that's not what you want.
Portal's saves are stored locally, in C:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\SAVE . If you find a completed save file, you can copy it into that directory and load it. This will unlock everything.