Not able to login into my dashboard after a migration to host

I recently did a site migration from one host to another. The site looks good, but the login does not work (/wp-admin or /wp-login.php). I get a Server 500 error. I need to do some work on this site before bringing into my multisite.

I can't log in at all.

Any suggestions? Or can you recommend any articles that might help me troubleshoot? So far, the host was not very helpful. They did say they found a "hacker" file called wp-in.php in the WP Includes folder. I deleted it, but that didn't help me log in.

Thanks, Joe.