Does anything come on the screen at all or is it black with just the power light on but no Hard Disk light lit. If that is the situation
1. remove the power lead
2. remove the battery
3. Hold down the on/off witch for 15 seconds
4. reinsert the battery and the power lead
5. turn on as normal.
You should now be working. Happened to me on Monday and that cured it, boots up as normal now