An easy and inexpensive thing to try is to replace the CMOS battery. It's likely time to do that anyway. A dead CMOS battery can make a computer appear to be dead - this happened to me. When I replaced the CMOS battery, the computer came back to life.
If that doesn't fix it, you've lost only about $5. And you probably should have replaced it anyway, even though replacing it didn't fix this particular issue.