Not strictly a Firefox question, but let's give it a burl.

If everything is *exactly* as new, then something has probably triggered the machine's factory reset function. If just the most recent files and Windows settings were missing (but some files and settings were still different from the as-new state), then this could be an accidental Windows rollback, which might not be a catastrophe.

A factory reset will wipe the user and operating system partition(s) of a drive and replace their data with factory default data from a hidden partition or other such source. At this point the old data is overwritten or left for dead. The latter can still be saved, and even some overwritten data may not be fully gone.

First, stop using the drive right now to prevent further damage. Then you have a few options to restore old data:

1. Backups. Everyone should keep backups - a single electronic copy is easily lost, corrupted, destroyed or otherwise rendered unusable.
2. Recovery software. I have had success with EaseUS, http://www.easeus.com/datarecoverywizard/free-data-recovery-software.htm, and Recuva, http://www.piriform.com/recuva. Both work best if you can access the drive with another computer, so it is not working as an active system drive.
3. Professional recovery service. A quick search should tell you if your country or area has professional data recovery services available. They cost a lot, but may salvage an otherwise hopeless case.

PS. When you say "logged in to your computer", was your old user account still available? Or are you using a default account?

It's a default account. I'm trying the recovery software you suggested. Thanks for your help.

Okay, so even the account has probably reverted to the as-new state. You're welcome, and fingers crossed that there's still something left to salvage!