Sometimes your site starts having problems and you know it is a plugin causing it. Other times it starts having problems and, since you haven’t changed anything, or maybe you even did change something, just just don’t know what is causing it.
Plugins are the single most common culprit.
The first thing to check is to simply make sure your plugin is up to date with the latest version available and is compatible with the version of WordPress you are running. You can always lookup your plugin at the plugins directory to learn more, or check the plugin author’s page.
If you aren’t sure if it is a plugin affecting things, use any FTP program or your host’s provided tools, to access your file system and go into wp-content, then rename your plugins folder. This will automatically deactivate all plugins(with some rare exceptions). You can then check out your site and find out if the problem is fixed or is still occurring.
*Note that some plugins store files directly in the wp-content directory, wp-super cache is one of these plugins. If you have a cache plugin, make sure to rename all the cache-related files in wp-content as well, or else you may just end up seeing the same cached page.
If renaming the plugins folder fixes the issue for you, you will then need to rename it back, and go through your plugins one by one, testing to see which plugin(s) are causing the problems. Once you know, you can try deleting and reinstalling the plugin from scratch. If that doesn’t fix it, make sure the plugin is up to date. If that doesn’t fix it, you will need to either go without that plugin or look for a suitable replacement plugin in the plugins directory.
If you really like your current plugin, you will want to go to the plugin author’s page and offer your experiences in a comment to the author. Perhaps the author will already be aware of the issues, or will be able to look into it, and let you know how to fix it.
Upgrading WordPress is a common cause for plugins failing, as plugins were made for certain versions of WordPress and are not always immediately compatible with the latest versions.
Before you install a plugin, it is a good idea to visit the plugin author’s site and make sure there are no special requirements for uninstalling that plugin. Sometimes just simply removing a plugin isn’t enough to properly uninstall it.
If you changed your theme to add code for the plugin, that code will likely also need to be removed.
Finally, if problems have become so bad that you simply cannot get things to work, perhaps you have a corrupted database etc., you may need to simply restore from that backup which you most certainly have since you are surely following all the backup recommendations offered all over the WordPress Forums and other places, right?
If you need to setup automated backups, see: http://wordpress.shadowlantern.com/2010/09/how-to-automatically-backup-your-wordpress-database/
If you have suggestions or if there is something important I missed, let me know!