Hi,
According to the compatible RAM upgrade for your laptop yes it can be upgraded up to 8GB max and you need a DDR2-PC2-6400 1.8v RAM type in order for it to work. Please do verify if the RAM that you upgraded it with is this type. Do also make sure that you have the latest BIOS version installed.
Here's the link for the latest BIOS version:
http/ftp.hp.com/pub/softpaq/sp52501-53000/sp52622.exe
Hopefully this will help.