What SSH Commands Help Diagnose Downtime?

Like most customers with a VPS we've experienced downtime. Of course, when our websites go down we'd like to gather as much information about what's causing the downtime as possible so we can prevent it from happening again.

Once we log in to the server using SSH what commands do you all recommend running to determine where issues are coming from?

Also, are there any resources you would point us to so we can learn more about how to handle downtime for our websites?

Thank you.