HELP! Did a PRAM reset and OS downgraded, lost passwords, desktop folders and files and dock apps.

I Did a PRAM reset because my mac was running weird and now everything has changed. All the files on the desktop have disappeared, the apps on the dock as well, the appearance of Finder is different, some applications don't work because they are for older OS. Seems like the OS was downgraded. I logged out and can't log back in with my Admin psswrd even though the hint that I get leads to that password. I dont xknow what to do and have lost a lot of proyects becauae the plugins I had are not installed.

HELP please!?