A filesystem goes into read-only mode when it has consistency issues. It is a way to prevent possible data corruption. Your next would be to take a backup of all important data from this drive since this could also mean that the hard-drive could be on its way out.
When you rebooted the machine, the
/ partition got mounted back in the regular
rw mode from the read-only it had gone to before rebooting.
You must run an
fsck on the
/ partition to check for any inconsistancy. Since it is the root partition, I believe you will have to get into rescue mode to run the fsck.