When you install a modification (mod), you, being a good admin, will backup both your database and your forum. BEFORE YOU INSTALL ANY MOD!
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 backup, for you.
Trouble is, restoring this backup will only work for the very last mod that you installed.
(You CAN use it to restore older mods, by going through the files in reverse date order, to be honest, but that's a bit involved)
Anyway... How do we restore the backup?
Using your FTP program, have a look in your forum's "Packages" directory.
In there, you'll find another directory named, you guessed it, "backups".
Now, let's assume, for the sake of this tutorial, that the last mod that you installed was "PM On Registration".
In the afore-mentioned "backups" directory, you'd need the file:
The numbers at the front of the filename are the date that the mod was installed, so they'll be different, for you.
Download that file to your desktop, or somewhere fairly empty of files and dearchive it.
It's a "gz" file.
When you dearchive it you'll be left with a "tar" file.
If you dearchive that, 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 should be exactly as it was, before you installed the mod!
Next time, backup before installing mods, won't you?