First you need to look and see if the actual files are still where they need to be, if you did a default install, your client files should be in this folder when you look in Windows Explorer. 

If you see all the files in that folder, then you may just need to do a Homebase Maintenance to bring them all back into  HB.  You need to be in a regular HB view (not the EFCenter) to run Maintenance

(1) It depends.  If cleaned up and update virus protection and drivers, the files are still there as Just-Lisa-Now said.

(2) If used "bleach-bit" like Hillary Clinton did, then the files are gone......

(3) If you don't have backup software (physical or cloud), get it.  I've heard that computers sometimes break........