Make sure you have a speaker or speakers or the equivalent connected to the mboard so you can hear mboard beeps (see your mboard manual if you need to).
Remove the AC power to the case/power supply.
Remove all the ram.
Restore AC power.
Try to boot.
If nothing else is wrong, you will get no video but you will hear a pattern of beeps that indicate no ram is installed, or a ram problem.
E.g. for an Award bios or a bios based on one, that's often a beep of about a half second, silence for a half second, a beep of about a half second, silence for a half second, continuously.
If you get the beeps expected for no ram is installed, or a ram problem, there's probably nothing wrong with the mboard - you probably have one or more ram modules that is/are incompatible with using it in your mboard - that's a very common problem - or much less likely, one or more ram modules is damaged.
Where did you get the ram from? If it wasn't installed in the mboard before it isn't necessarily compatible with using it in your mboard.
Ram that worked fine previously should still work fine, unless it's been damaged by an external event.
NOTE that some HP computers have a BESTEC power supplies - it that's what the brand of yours is, they are well known to malfunction more often than most power supplies, and when they fail completely they will often damage something else, often the mboard.