Grid status popped an announcement about 11:30 AM SLT/PT that regions are crashing and dropping offline. The problem started over the weekend. The Lindens are working on it.

They say:  If your region is offline, please go to your support portal and submit a case using the following types:  “Land & Region” > “Report an Offline Region”.  This queue is being closely monitored to ensure the fastest turnaround possible. 

The post is here: Ongoing Issue with Offline Regions