What model is it? HP did a whole bunch of terrible laptops a while back that died with black screen/lights on but nobody home being one of the main indicators of that. Ones featuring Nvidia graphics chips being the main culprits here, to the point there was a massive lawsuit on the matter and while if you go looking up models that you can claim for then know that is a somewhat narrow list (the same chips, the same motherboard designs, same thermal profiles... got used in other models so yeah). Such a failure is likely beyond any economical repair, and even if you did find another motherboard of that type* then the same failure model applies there as well.
Beyond that I would probably consider it toast. Rip the hard drive, RAM and whatever else out of it if you want but if water did get in and has been sitting underneath a nice BGA chip then the effort required to fix it or tool up to fix it is likely far more than you could swing some kind of refurb laptop (possibly of better specs) for. If it was just a dodgy keyboard, bad power supply or supply connector, ram not seated properly, hard drive that had given up the ghost or similar then than is one thing but unknown water damage over a long term, for a laptop maker with many notoriously unreliable laptops... there had better be a very good reason or some seriously nice hardware in that thing.
For a sanity check though does anything change if you wiggle the screen back and forth (almost closed to fully fully back). This is mainly to see if it might be screen ribbon issues (sometimes laptop makers made them a bit short and that caused them to snap eventually, moving it closed to open fully sometimes then gets it all to connect again). You might also want to take the RAM out and put it back in again aka reseat the RAM -- if it got knocked really hard it might have knocked it out of place and failure to boot is often a result of that.
If you really want then you can take a look at the motherboard to see if there is anything obvious (nice green corrosion or obviously blown chip, maybe even probe some fuses or fuse stand ins) but given how annoying most HP laptops I have ever dealt with to pull apart are then have fun with that, and even if you do find something then while I don't do fly by night repairs if I am to fix such a thing for someone then the warranty is "the second I walk out of the door" in the scenario you describe.
*if you are lucky and it is not that type you might find a broken model on the usual suspects for cheap junk that you can swap parts around to make a working machine from.
