My PC power light is flashing flashing green but it not booting up, what is wrong?
This is a good guide on boot issues. Check everything on this list one at a time and double-check everything is connected correctly. Post back here when you're done and we'll go from there.
It could also be a memory issue.To test faulty RAM try using only one stick at a time and try each stick in each and every slot on the motherboard. This will rule out either bad memory or a faulty motherboard. Failing that, try removing all non-essential components and replacing each one by one to locate the faulty component. This includes everything except your motherboard, CPU and RAM.
If you've been through everything on this list and still have no luck then you're down to your motherboard, CPU and PSU. These can only really be tested by substitution - just swap each part one at a time until you find out which one is faulty. Hope this helps and post back to let us know how you get on.