Power button not working on desktop

I have a Dell computer I purchased in 2005.  It has worked great and I have had no issues.  All of this sudden, this morning, the power button won't work.  I push it and push it and nothing but a yellow flashing light (the button flashes yellow).  I unplugged everything and plugged it all back in twice and finally got the power button to work.  Midday, I had to shut of the unit for a bit-when I came back to start it again, had the same issue, except this time no blinking light.  Did the same thing, and finally got it to power up.  ANyone ese had this problem or have any ideas why this is happening???