As mentioned in the comments, the first place to look is the error log. That will usually point you to an error in a plugin.
You can disable an individual plugin by renaming it’s folder. You can quickly disable all plugins by renaming the plugins folder. If you rename the entire plugin folder, then create a new plugins folder, and move individual plugin’s folders into the new folder, thereby enabling them. The ‘bad’ plugin will cause the error to reoccur.
It is possible that a theme is causing the problem. Use the same technique to quickly disable your theme…that will cause WP to use the default theme.