These are some basic troubleshooting techniques for WP ailments, in rough order of how frequently I make use of each.
Flush all caches: I am working on my site but the changes are not showing up. This is commonly a caching issue. I need to clear all caches, which might include Cloudflare, browser cache, caching plugin, and/or server-side cache provided by my web host.
Switch to a default theme like Twenty Sixteen: A theme conflict can manifest itself in many poltergeist-like ways. Whenever something strange is happening I switch temporarily to Twenty Sixteen to see if it exorcises the problem. One example I encounter frequently is failure to upload images to the media library. I have to switch to Twenty Sixteen, upload my images, and then switch back to my regular theme.
Disable all plugins: Similar to a theme conflict, a plugin conflict can cause various strange errors. I encounter theme conflicts more frequently, so I try that first. If it does not fix the problem, then disabling all plugins is the next step. If the problem goes away, I reactivate the plugins one by one to find the culprit. Unlike my theme, which my site is designed around and cannot be easily replaced, I can usually find a substitute plugin if one misbehaves.
Use SFTP or the host control panel file manager: So what if the error prevents me from logging into to my admin dashboard? I can still switch to the default theme and disable plugins, using the file manager in my host control panel, or an SFTP tool like FileZilla. I navigate to the /wp-content/themes/ folder and rename my active theme’s folder – just put an ‘x’ in front of the folder name or something like that. My site will not be able to load my usual theme, and will revert to the default theme.
Similarly, I can rename the plugins folder or the individual plugin folders to disable plugins.
Refresh Permalinks: A glitch in the WP permalinks functionality can cause 404 ‘not found’ errors. I can refresh permalinks by visiting Settings, Permalinks, and clicking ‘Save Changes’ without actually changing anything.
Last resort: On occasion I have had to restore from the site backup that I either wisely or luckily made before the troubles started.
None of the above work? Try asking in the WP support forums.