I'm not sure what update you wanted to install but try pulling the battery and see if your phone will boot. If not, then boot into recovery and restore your backup. Once your get up and running I would verify that the Dmupdater is a solid file and available for installing.
My Droid did the same yesterday. I updated thru the ROM Manager to Froyo 2.2 and after it installed and went to reboot, it has been stuck on the DroidMod screen ever since (3pm yesterday). I cannot get to any other screen.
I have tried: removing the battery, removing the sim card, using another battery, holding down the camera button, on/off, volume up and down buttons at the same time. Plugging into my computer, computer won't open/recogonize the device. Nothing is working. The screen is "refreshing" but won't update.
Any other ideas? HELP, please!! FYI - I'll need layman's terms...
What I ended up doing was going into recovery mode and did a backup and than a data wipe and it rebooted and has worked fine I had trouble with the camera not working but after rebooting again it worked fine