Try plugging it into an external monitor. If this works, it's probably the screen - if not, likely your video processor or drivers. Hopefully the latter, as you can just upgrade them from the HP site.
Assuming the machine isn't still under warranty, yes you can buy a screen (Google/eBay). HP are pretty good about making their repair manuals available online. If you can't find it, I'm pretty sure that I have one for a 6120 (we have a few in the schools I work at).
Before buying a screen, I would check that there aren't any loose connections. From memory there is a cord that runs from under the plate at the top of the keyboard or possibly under the keyboard that sometimes works loose. (The latter is easy to remove, I'm pretty sure that the former takes a few more steps). It then connects "inside" the screen. It's not too difficult to get to if you're OK with a screwdriver, but I wouldn't recommend it to everyone. Maybe take a look at the manual and see if you're up to it.