Cpu run but booting process doesn't occur

Hello,
hi,
my computer
has some problem from last 5-6 days.when i start my computer then some time after booting it reboots again & again. CPU light is continuously on yet it restarts again & again.some times CPU green light is ON but still it is not showing booting process means monitor has no output. & CPU fan speed becomes very fast.i think there may be some problem in cable Between Monitor & CPU.but this cable is OK from outer & also connected properly with CPU.Then what can be the problem please help ......