|You didn't mention your ram. Ram that is not compatible with your mboard can cause no boot - did you add to or change any of the ram in the original mboard just before this happened on the old mboard? Did you transfer the old ram to the new board - if so, it may not be compatible with the new mboard.|
"I have switched the power supply"
Are you sure it works, and has enough capacity?
Have you tried the original PS on another computer? Did/does its fan spin properly?
Can you smell any obvious burnt wiring/burnt plastic kind of smell at it's external fan outlet when it is off (if your smeller isn't so good, ask someone else who has a good one to smell it)?
A failing power supply, especially if it fails slowly, can damage things connected to it, and unfortunately what damage it does is random. I've personally seen a few worst cases. One killed a floppy drive and a CD drive, and after replacing it I later found the modem card didn't work properly anymore, and the video card died within a short time - the mboard, ram, and cpu were fine. In two other cases it fried the mboard - in the one I have I haven't been able to test the cpu, but the ram, CD drive, and hard drive was transferred to a new mboard and works fine (the original mboard had onboard video).
I've seen several cases where failing PS's kill the floppy drive first - that is a red flag that the PS is dying.
You didn't mention your video card. If your new mboard is using your old video card, and you haven't tried a different video card in either mboard, a dead video card will cause no boot on most computers.
"Then I got a new processor"
Are you SURE it works in the new mboard - CPU support tables often only apply to faster or later added cpu's if the bios version you have is new enough.
Other than that you've already tried things I would have suggested you try.