All files lost, NEED HELP ASAP

[Anna]

I'll admit right away, I do not know enough about computers to figure out what happened/what to do.
After turning my macbook air on a few days ago, I could not log in and instead had the Mac OS X security screen (it was gray) that asked me to connect to a wi-fi network and then to enter a 4-digit passcode. I had no idea what this was, didn't know the passcode obviously. Later I found out that my sister knew it, so I entered the passcode that she told me, and was able to log in to my account. HOWEVER, everything was gone: documents, iPhoto, itunes, microsoft office, bookmarks, etc. Basically, my user account was wiped clean. I do not live with my sister, so i have no idea what she did, but according to her, she tried to remove her iphone from our home iCloud the day before all of this happened. She went on icloud.com and removed her phone from the Find my iPhone thing. that's all she knows.
I went to my local Apple Store, and they were not able to help me. I ran a data recovery software (Disk Drill) and didn't find anything (the only files recovered were stupid tiny .png pictures and other useless crap). Please, I need help asap. Needless to say, there are some extremely important documents that i need for work, research, and med school applications!!!

 

[Kaveman]

In System Preferences > Accounts what accounts are listed?

 

[Anna]

Me as the current user and admin, and guest user

 

[Morhass]

Sounds like your sister has accidentally done a secure wipe on your Mac via iCloud, instead of whatever she thought she'd done. Apple's security stuff is really thorough, so you're unlikely to get your files back from the computer's internal storage...
That gives you two options... 1) if you had iCloud Documents and Data turned on and were saving your documents there, you should be able to reconnect to them. 2) Time Machine is really awesome backup software, it makes it really hard for you to lose important data. Connect the backup disk or Time Capsule you were using for your Time Machine backups, and restore your data from there.