Why does server shut down after backup?

The NT server at one of our branch offices shuts down after doing its nightly backup. By shut down I mean ‘off’. The server itself acts as if somebody turned it off via the power switch. When the computer is turned back on the system log reads that the backup worked and everything seems fine. Problem is this is a branch office too far away for me to visit, it’s really scaring the not-so-computer-literate manager at the site. I need to be able to ‘fix’ this over the phone. What should I tell her to do to make this stop?