My server will not reboot!

I came in this morning to find that the air-con in the server room had failed. Three of the four servers there seem to be fine but one will not reboot having shutdown. When restarted it runs POST, memory checks are ok, all three RAID drives are detected and seem to check out but then I receive a message saying ‘boot device not found- Press F1 to reboot or F2 to enter Setup Utility. The server is a Dell Poweredge 2550 with three 40GB RAID HDD. Can anyone help?