1. Try booting in to Safe Mode by holding down the shift key when the system restarts. If the system is able to boot properly, then it is likely a third party extension or some add-on that is causing the issue.
2. If you can't boot into Safe Mode, try booting into installation disk that came with your computer or a Snow Leopard retail disk by holding Command-R at startup. If the system is able to boot properly, then it is likely a problem with the kernel extensions and you will need to reinstall your OS.
3. If the system doesn't boot into Safe Mode or into your installation disk/retail Snow Leopard disk then you probably have a hardware issue. At this point you could try resetting your PRAM and SMC to see if that resolves the issue. If not, you will need to take it in to be serviced.
OS X Yosemite: Reset your computer’s PRAM
Resetting the System Management Controller (SMC) on your Mac - Apple Support