Recently came across an interesting scenario from one of our colleagues for our clients, when both the instances for online environment (sandbox as well as production) were inactive:

I never witnessed such a situation when both the instances were supposedly down and first thing came to my notice is to check if CRM online was working fine for others, which is definitely the worst case

possible because we have amazing team Dynamics Engineering at Microsoft keeping CRM up and running at all times.

I checked with few people in the EU region and it seemed to be up and running. I was a bit surprised.

A little querying with CRM experts made me suspect that there are definitely some places where we should be able to check it.

Often such scenarios are caused by inactive licenses for customers. The place to check this in in Office 365 admin centre under billing:

If you see such an issue, more often than not it should be related to office 365 licenses: which you can check as per above screen.

Hope it helps!