Most Common Troubleshooting Techniques: (without getting into too much detail)
#1. Rename your plugins folder - If a plugin is the problem, renaming the folder where WordPress looks, will cause WordPress to not load any plugins inside that folder.
#2. Changing your custom theme to a default WordPress theme.
#3. Check for caching configuration issues. (cashing plugins)
#4. Troubleshoot any possible database issues.
If you had no success, try updating or even going back a version of PHP running on your web host. One thing we must remember, not all versions of PHP are guaranteed to play nice. Just because you upgrade to the latest and greatest doesn't mean it will perform any better. In fact, it may perform worse, or in my case temporarily take down all 20 of my sites. cause a white screen of death, on not just 1 website, but 17. Scary stuff. I was fortunate to get my sites back up within an hour. This isn't always the case.