To solve this you need to know what changed since the last time you successfully logged in. 500 error is a server side error and the first thing that comes to my mind is a plugin issue.
I would advise first to manually disable all plugins and work out your way from there.
You can follow the instructions in this video to manually disable all your plugins when access to the WP admin is not possible:
I hope this helps! If it does, please hit that Kudos below 🙂
If you are on one of the managed wp platforms that have automatic backup you can also restore to a day when you know the site was working. From there work forward with your theme/plugin/data updates until it breaks. Then you know! Its a Pain-in-the-rear practical way to troubleshoot without having to start digging too deeply into log files, etc. Those are a bit intimidating when you're just getting started.
There are tools like WP Theme Checker that you can use to get insights into your site's health. This may be a tool to start with. https://wordpress.org/plugins/theme-check/
Hope this helps,
I think you will need to contact your hosting provider, this doesn't seem to be an issue of Wordpress and it can't be resolved without looking on the server logs and verify what is causing the 500 error.
Good luck with this.
I hope this helps. If it does, please hit the Kudos below 🙂