This could be any number of problems, possibly (but unlikally) even a PSU problem.
First of all, stop trying so much
If it doesn't work after a few tries, the more you power it up the more chances you will damage it further.
Does it work when you try and run it in safe mode? If it does, get in and run a checkdisk. You might have damaged the master file table or the master boot record; which leads me onto my next point:
IF at anypoint you manage to log in, run a virus scan and anti-spyware scan. If you don't have either, i don't know what most people on Computer Hope recommend, but i personally use AVG and Spybot S&D. Download them from another PC, stick em on a USB stick, plug in and run.
If that fails:
On startup, try loading the BIOS and going into the PC Health section. Take a look at your Temp, if it's sitting at around 30 degrees, its fine. Find the option to set an alarm, and set the warning temp to about 60 degrees. If it's an overheating problem, you will know.
Check all your cables are plugged in correctly. Power and data cables to harddrives, cables to the motherboard, everything. Make sure any additional cards and the memory are seated correctly.
