This has become a common problem with the old models GO 700 and GO 910. They both have a physical hard drive (which contains moving parts), as opposed to a flash drive. The GO 700 has a 2.5GB hard drive, whereas the GO 910 has a 20GB hard drive. The life expectancy of these drives is not much more than 5 or 6 years, so it's quite possible the hard drive has died.
There are a few things you can try to fix your problem. It has recently been discussed in the following thread:
TomTom GO 910 - can´t start