Richard,
We need some more details.
Are you referring to boot-up failure?
Can you access the boot options screen (F8) during boot to get to safe mode?
Best option is to disconnect any 'usb' peripherals and try to get to safe mode. Failing that see if you can get to the BIOS settings to see if your components (CPU, RAM, Power Supply, Drives, and Monitor) are listed and whether the listings are correct.
Failing the above, try opening the case and remove all but the first bank of RAM (usually slot '0' in a two slot MoBo, or slot '0' of channel 'A' in a four slot MoBo), disconnect all drives except the drive with the OS installed, and check all connections (including the power supply connectors, the drive connectors, and reseat the one stick of RAM).
Then try to boot to safe mode (or normal boot if you like).
Some basic ideas based on the little current info. 