If she is not getting any error from the OS, like missing file..., it's usually a hard drive failure. Which you will get a message like no operating system found, or not boot device.

If you can do a little more diagnostics we can help you more.
Check the BIOS, sees the hard drive and is set to boot to it.
If you can get in the computer, check the cables are not loose. The hard drive is not unusually loud. Download and run a hard disk test utility.

Here's a link to the Ultimate Boot CD. You can boot and run several diagnostics tests from it.
http://www.syschat.com/download108.html