With one Wordpress site I had big trouble lately. While I put the site on "Maintenance", I tried to update some plug-ins, und there it happened. My site was stuck in this maintenance mode, and I couldn't access the admin area. It coasted me hours to find a solution. Deleting the .maintenance-file via cPanel-File Manager (or FTP client) did not help. Disabling the theme neither. Then I tried to disable all plug-ins, and then one-by-one counter-checked, and there I found the culprit. One of the plug-ins has been corrupted during the update process. I deleted that plugin, and could access the admin area finally. I think, a lot of Wordpress users faced this awful situation. I needed to share my agony, herewith:)
Not sure whether this is what happened to you, but I found that the worst thing one can do is when WordPress plugins are in the process of being updated, to interrupt the update while it is happening. It can be by accident by clicking on something. WordPress is very unforgiving when one doesn't allow the update to be completed first before one clicks anywhere in the page. I learned it the hard way once. Fortunately no harm came, but I had to wait maybe for hours to be able to get back into the dashboard. When the updates are in process, it's the last place one can afford to be impatient with the process completing. I need to see that wording at the bottom of the plugins that the update has been completed before I do anything.
1. rename .htaccess and create a new one with the default WordPress rules / if you still receive 500 go to step 2
2. rename plulgins folder / don't forget to clear your browser cache or try with a different browser - if you still see 500 go to step 3 if not there is a plugin issue
3. if you have php.ini or user.ini file rename them (this way you will disable them)
4. check what is your php version (you can try with a different 5.6, 7.0, 7.2)
5. Do you have error log? If "yes" what it say :)?