please help regain admin access

I posted a couple days ago. My site is swarming with malware. I took it down ofr a while (8 months or so) and when i reloaded WP files i do not have admin access, the login page does not load. (error 500)

Several people posted much helpful info about virus/malware protection but that is all moot if I can’t get in to install plugins and whatever other steps that might be recomended.

the site is oldrv dot net but it is blacklisted so there is no reason to go look there.

I just want some clues as to what WP files might be messed up and causing the falure to load hte login page.

Thanks

Dave