This could be anything, including the BIOS stopping on reboot due to a simple error such as a keyboard being unplugged. (I know from your post that this is not the case, but some BIOS are set to stop on "any error".)
First question is did she move the computer recently or recently install a new printer? I'd check all external connections. If that does not work, I would open the case and apply finger pressure to make sure the hard drive and power cables are connected, memory is not dislodged, etc. I would then leave the case side panel off and make sure the CPU fan is spinning when the power is turned on.
Upon turning the computer on, have her immediately start tapping the F8 key at one second intervals. See if this will bring up the Windows option list to "boot in safe mode" or "boot with last known configuration that worked".
Is she running MS Vista? Sounds probable based upon the age of her machine. I do not care what anyone says, Vista still has problems at times, especially after installing patches / updates Microsoft pushes once a month on their "Patch Tuesdays". I also know what week had Patch Tuesday as I always get a call that one or more computers, especially one running Vista, is having issues on Wednesday or Thursday of the same week.
Please keep us informed as to how she is making out. I'll be glad to try to help.