AMD athlon wont start up

I have found on MANY socket A motherboards, that you need to set the CPU front side buss and multiplier manually.

The trick I use to recover from the form of boot lockup that you have is to:

Remove the CMOS battery
Un-plug the power supply for at least 1/2 hour.

Connect power, and boot up.
Enter BIOS settings, and set the CPU and other settings.

Save changes.
Re-install the CMOS battery.

It is also possible that the cause of your lock up was the CPU died.

I have 3 dead socket A CPUs setting on my bench.

Chas

I would also double check the mounting of the motherboard. There may be a standoff that is mounted in the case where there is not a mounting hole on the motherboard. This can cause an immediate ground or even an intermittant grounding of the motherbaord. Result, no power up.
As far as socket A chips are concerned, I have not seen any higher fail rate for the AMD than I have for the Intel. This is for the retail versions of AMD only. The OEM and bulk AMDs are a different matter. Stick with the retail versions, they have a 3 year warranty from AMD and a low fail rate. Intel, on the other hand, does not seem to suffer with OEM and bulk chips the way AMD does.

I realize that this may not pertain to your issue, but it is good knowledge for future reference. I hope this helps.