The Virtual Machines on one of my Data Stores are getting IO errors when trying to start. Any Virtual Machines that are still up have all gone read-only. What is going on?
This is most commonly seen when there is a connection issue with the SAN. Checking the status of ISCSI sessions or whichever storage you are using will often reveal the source of the problem.
/var/log/messages on your Hypervisors should give some clues to the cause of the issue.