There’s a lot that could be going on, so it’s hard to say if it’s a theme issue or another plugin causing the problem.
What you can do is open your site up through an FTP browser. Then, you could find your
wp-content/plugins folder and create a subfolder named
temp. Drag all plugins into that temp folder, then try to log in. This is essentially disabling all plugins, so if it’s a plugin conflict then this is one workaround. You would then drag the folders back to the correct place until you find the one that is causing the problem, or you could re-install plugins (settings won’t be lost with this method).
If that doesn’t work, you’d want to do the same thing with themes, making sure you have at least something like Twenty Seventeen or a default theme enabled.
start with the following:
-be sure you have a recent backup of your site files and database
-deactivate all plugins
-switch to twenty seventeen theme
-set wp_debug to true to see if any errors show