Internal Server Error just when about to upgrade WP

Hello everyone,

I logged on to my site (https://hsponline.org) and noticed the Update notification in the toolbar. I clicked the Dashboard menu and WP froze for a while and then logged me out.
When I tried to log in again I got an Internal Server Error.
Now I cannot log in no matter what I do – clear the cache, close browser, restart browser, use different browser… still see Internal Server Error.

I just downloaded a copy of the whole WP folder via FTP and also exported a backup of the SQL database from my hosting interface (GoDaddy).

Anyone have any ideas what the problem might be and how I can fix it?
I was looking at manually upgrading WP but am not sure how to deactivate plugins via FTP.

Help, please!
It’s a live site with users actively submitting applications daily.
Any assistance is highly appreciated.

Thanks a million!