Hi please help

I have an hp evo d510. When I power on the desktop the led indicator is just blinking... my PC doesn't start up... any idea?

Just a quick thought, try removing the power lead from the computer for about 5 minutes, and re-inserting it. I've seen some branded machines that go into a kind of safe mode after a small surge on the power system. The only way to make them start is to cut all power and allow them to reset.

Sounds like it has gone into standby mode.
As Stu says, pull the power and then reboot.
Once you get in check your power settings.
I don't use either standby or Hibernate as I find them an annoyance, but then again they are not really recommended for a laptop anyway ;)

I believe HP uses the leds (power and hard drive) as a trouble shooting tool
Check HP website, I think you will find you have a bad system (mother) board
Or so they say, sorry for the bad news!