Hello Ron - The two most likely possibilities are that you either did not seat the CPU properly in it's socket (misaligned or bent pin) or that you did not fasten the heat sink properly, or remember to plug the heat sink power connection into the motherboard power connector.
With the heat produced by modern CPU's - without the proper cooling they would melt down almost instantly that's why both Intel and AMD have built in overheating protection which keeps them from even starting if not cooled correctly. When replacing the CPU it is always good practice to apply some new thermal paste after cleaning off the old.
I suggest you try starting the machine with the cover off and see if the heat sink fan is turning. If not that's at least part of your problem. If it is turning I would remove the CPU (careful of static!) and examine the bottom carefully to make sure nothing looks damaged. If it looks normal I would try reseating the CPU slowly and carefully.
In your question you did not say why you removed your CPU. Where you upgrading it to a new one? If so are you sure it is compatible with your motherboard?
Here is a link to HP's info page about your motherboard, including upgrades. Click Here.
It's also a good idea to clear the motherboards CMOS when removing and installing a CPU. The same page has a clear description on doing that.
Please follow the steps as I've presented them below:
1 - examine the info at the link I presented.
2 - make sure the fan is working - If so then -
3 - remove the CPU, examine and if it looks OK replace carefully (It should take hardly any pressure for a CPU to fit into it's socket.) Thermal paste would be nice but there's probably enough on the CPU (unless you've wiped it off) then you would need to get more paste before proceeding.
If that list doesn't help then it's time for a call to HP/Compaq tech support.