I had no idea that our version was so out of date!
When the database is okay, but some SMF files are damaged, it may be a good idea to reinstall the current version of SMF. This recreates what is called a "clean SMF installation". This might be necessary if:
A mod fails to completely install or uninstall, leaving the forum unworkable
An update/patch of SMF fails because of the changes mods have made.
Instructions to restore the current version of SMF, without making any changes to the database, are listed below. Once a fresh set of files has been installed, it will be possible to re-install mods or upgrade to a newer version of SMF. Following these instructions will delete all the edits made by mods and other changes to the SMF supplied themes, and erase all hand-made customizations from all files. Following these steps will not delete any "extra" files or directories you may have added by hand or through mod or theme installations. For other options, please seek help in the 1.1.x support board or the 2.0.x support board.
Back-up everything
Back-up your database, ensuring that you have backed up all your data.
Back-up your custom themes and graphics. Make careful notes about any hand-made customizations you have made to files.
Optional step: Warning: do not undertake this step unless you know what you are doing! Before you upload the fresh set of files, you can also remove Sources and Themes folders. Doing this ensures there are no files left behind and that your setup is completely clean.
Download the large upgrade archive file for the exact version of SMF that you are using from the download site. Be sure to select your current version of SMF from the drop-down list next to "Select Version". (Please note: You should NOT be downloading/using the full install archive file.)
Extract the archive to the forum's directory, making sure that you delete upgrade.php and all the upgrade-x.x.sql files, and overwrite index.php, the sources folder, and the themes folder. You can either use FTP or cPanel's File Manager. On your forum, if you have replaced the standard smiley files with your own and/or replaced the standard avatar files, you might want to refrain from uploading both the smileys and the avatars directory. Important Note: If you carried out the optional step, you should also upload the sources from the large upgrade archive before you reinstall any custom themes.
Once completed, your forum will be in a virgin state, software-wise, but all of your members and posts will be intact. Your mods, while no longer installed, will still be in the Package Manager. From there, you can either apply/install them, or delete them, as you desire. Any of your mod's settings that are stored in the database will be retained and still be usable when the mod is reinstalled.
If you are using SMF 2.0 and had mods installed you may want to use Fix_Packages.php to remove the hooks that the mods installed. This is a good idea if you see some mods are still installed when browsing the admin panel after uploading clean files.
Remember: for the best-performing and most secure forum, you should plan to patch or upgrade to the most recent version of SMF as soon as possible.