Hp pavilion will not start need help to figure out why . Itook out power supply and had checked at store they said it is ok

Ah boy, so many things could be wrong, but usually, it's something simple, but tricky, like this problem. My guess is that it still could be the power supply- it depends on how they check it. If they just measure the voltages, that can be misleading. Depending on the type of power supply, a supply line can, under the load of being hooked up, drop just enough to not function good enough. The solution to this problem is to replace it with a known, good one. If the problem's still there, then it's either your motherboard or a component that's shorting out the line. If you try a different power supply, and it's still not coming up, disconnect the CD, DVD, and any accessories to try to isolate the problem. If only the motherboard is left, you will have to have someone troubleshoot it, or replace it. A used computer store might let you try another power supply on it for free, in the hopes of selling you one, and then you would at least know the problem is with the motherboard, and they might be able to check that out for a resonable price.