my preamp wont power up

Make sure you have power at the AC outlet. Sometimes those outlets are on a switched circuit. If so, flip the switch.

Also, if it has a detachable power cable, make sure it's plugged all the way in.

More info would be helpful. What make/model of preamp?

Thanks,we have power.Im wondering if its the micro proceesor.The pre is a ar reference 5

Can you look inside and see if the tubes are lighting up, or is the unit absolutely dead? I'm pretty sure you can turn the screen off.

You can turn the screen off on the Ref 5. But there is still a green dot lit in the center. The screen default is on.

If it worked when it was unplugged, my guess it is something simple. Unless you kept it in a bathroom I doubt the humidity is the cause. Is the power cord pushed in all the way? Are all the tubes seated properly? Is the 6550 in the power supply lit up? Do any of the tubes light up? What exactly does it do and not do?