My son popped of my compaq presarios space bar key... help fixing it... I just ogt itback from the shop

Some keys are different. Here is a generic steps for a specific Prasario model. How to Replace a Keyboard Key on a Compaq Presario 2500 - MachinaElectronics.com

I've had a key or two off mine. In fact I just popped mine off. Hopefully the metal bail is still in place and there is no damage to the key.

Carefully align ALL the tabs at a 45 degree angle and press down until it snaps into place.

Note, that you can buy specific keys and the entire keyboard is easy to replace.