Server error

I have a test site setup that I’ve used several times in the past. Today while trying to use it, I keep getting this message:

Server error
The website encountered an error while retrieving https://test.fifty2ninetyonline.com/wp-admin/. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this webpage later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

I’m not getting an error when I go to the site (that is, the same url without the text. prefix).

This has been going on most of the day. Does anyone have any idea what might be causing this and what I might do to fix it?

Thanks.
Martin