The usual advice is to deactivate all plugins and revert to the default theme. If your problem then disappears you can put things back one-by-one until you find the culprit.
You can still deactivate plugins by accessing the site via FTP. Create a dir and drag your plugins into it. If you can access your dashboard then, you know that one of the plugins was the problem. Drag one at a time back into the plugins folder and see if you can still access your dashboard.
Remove from wp-content. I'm told WP is bright enough to cope with that. Never tried it myself.
Just a small addition. I'd put the plugins back in reverse date order of installation/activation.
If there are no new plugins, it could be that a recent update made an old one interfere with something. You'll get that once in a while with outdated plugins.