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 as to the cause of the issue.