Internal Server Error on admin page

Yesterday, I was able to log into the admin panel and send a campaign.
Today, people are able to access the sign up page and I’m getting emails from people signing up.
I can see the admin login screen and enter the credentials, and then it times out with an “Internal Server Error”.
I’m using 3.6.5 on a shared server hosted on DreamHost. I didn’t change anything - unless my system tried to auto-update to 3.6.6 last night.
Any suggestions what I can check? I haven’t been able to find any relevant ideas on google or elsewhere.

And it fixed itself? So never mind…
But if folks have suggestions what to look at for future reference, that would be great.

Internal Server Error? Check the server error logs, sounds like there was some down time. Unlikely to be phpList related I would have thought.