Sometimes after a XenServer restart you might find your host takes a long time (many minutes), to exit maintenance mode. This can be indicative of a problem syncing with the master server, but before you go about tears apart your XenServer install you should take a few minutes to see if it's running a disk check on one of it's storage repositories.
To quickly check to see if XenServer is busy working in the background, go to the host console and type 'top'. If you see an entry for fsck.ext3, it means you have an EXT storage repository that XenServer is checking. Sit tight and let this complete, your host will exit maintenance mode when it is done.
This is a simple tip but it can save you a lot of worrying while you wait for your host to come back online. If your host is using LVM storage repositories you should not have this issue.