When you install a modification (mod), you, being a good admin, will backup both your database and your forum files.
Some admins, though, aren't as clever as you are. So, they install a mod without backing-up first. Oh, dear. The forum's dead. I DON'T HAVE A BACKUP!!!
Well, relax. You MIGHT be lucky.
SMF, being the helpful system that it is, takes a file backup, for you. It does this each time you install something, and restoring the files will take them back to the way they were just BEFORE you did the install (if you had to, you could work through all of the file backups in reverse order to go back further in time, but that's a bit involved)
How do we restore the backup? Dead easy!
- you'll need to start up FTP or else the file manager that comes with your hosting service.
- you'll need to have a look in your forum's "Packages" directory. In there, you'll find another directory named, you guessed it, "backups". Suppose that the last mod that you installed was "PM On Registration", and that you did this on 26 Jan 2012. In the afore-mentioned "backups" directory, you'd find the file 2012-01-26_before_PM_On_Registration_1-0-2.tar.gz
- Download that file to your desktop, or somewhere fairly empty of files and dearchive it. It's a "tar.gz" file. A program like winzip can extract the archive.
- You'll find loads of files and directories. Upload those files and directories to your forum, overwriting what's there now. (It'll take a while).
When that's done, Voila! Your forum files should be exactly as they were, before you installed the mod! This procedure will not make any changes on your forum's database, of course.
Next time, backup before installing mods, won't you? ;)