OK. I did not know what you had, so I was just guessing.
The PC you got is worth about $125 on eBay.
You could try the HP forum , but you mint get more help here. We have a good bunch here tact can give you hints.
Unplug the power cord. Plug it in again and push the start button.
Do any lights come on?
Do you hear fans?
Do any lights on the keyboard flash?
You can get a used motherboard for about $50, but don't do that until you try a few things. Open up the case and see if anything came loose. Like maybe a RAN chip. Rare but it happens and it is easy to check.
The things is to check first the things that are easy to check, but seldom happen. You don't want to spend money and find out it was something else..
Other things easy to check.
Disconnect the power and data to the DVD drive. Maybe the DVD drive failed.
Same with the Hard Drive. The system can power on without the hard drive and should show some kind of error message on the display.
Again, even without the RAM present, the system can flash keyboard lights and make beeps. Wroth a try just to see.
Some vendors on eBay will sell the whole box for about the price of the motherboard itself. If it were me, I would buy the the box. unless I was really sure it is just the motherboard and not the power supply
Here is a document for that model.
www.manualslib.com... Hp Pro6300 Service ManualTake care when trying to download manuals. You do not have to buy anything and some sites that offer manuals are scams. The link above is alright, but they try to make it hard and get your to download something else.
Hope this is of some help to you.
The moderator might move this to hardware.
EDIT: The model you have might include options that can make it hard to service. There is a optional special feature to make it hard for somebody to steal you r PC or to use it without your authority. Read over the manual.