Restore username after turning on computer

Need to know what version of windows you are using. I am assuming you are using windows.

Windows XP> My computer > C: Drive > documents and settings > The name of the user folder you are missing > all your documents should be here

Windows Vista and 7 > Computer > C: Drive > users > THe name of the user folder you are missing > Same as above

If the folder is not here someone has removed it using the user profile delete option, if they opted to remove files when they did this your files are with them and system restore will not bring them back, because system restore does not affect user content. The only other way to get your files back would be a recovery software like "get data back for ntfs" which cost money or "recuva" which is free. Let us know what your found. Cheers and good luck.

One more thing, using data recovery software is not a guarantee mostly because any data that has been overwritten by windows will no longer be available.

Well, it sounds like a little revenge is in order.
1) Never let your kids (of any age) know your password
2) If you have a kid who is irresponsible (like many of us have), do not give them Administrative rights on your computer (instead give them their own cheap computer to mess up), give them a limited user profile so that they cannot do crap like this to you
3) After recovering your files (if possible), then lock his account down, and take away his access to his files for a little while and see how he likes it. If you are really vindictive then delete his files all-together. Life sucks when you have to redo your homework  

As for file recovery; There are a multitude of free and for pay software titles out there to do data recovery if they are truly gone. I would give EaseUS a try. Preferably you will want something that works outside of Windows and runs on it's own bootable CD, but with free products your mileage will vary. I ahve had some good luck recovering files from corrupt drives, and deleted partitions, but when something is on the system drive and it gets deleted Windows starts re-writing that space very quickly, so the damage may be irreparable.