Welcome to the forum
I also have a Philips (DVDR-5500, no HDD), and it can often be temperamental, though in all other respects it's a good unit.
Do any other buttons work - Play, record etc? If not it might have just frozen up, and unplugging for awhile then plugging it back in may fix the problem, as Jim suggested.
If not, then provided there is nothing critical on the HDD that you need to keep, then setting the unit back to the factory defaults might do the trick (this has worked for me in the past, though I've had to re-tune all the channels etc).
If you're sure there's nothing important on the HDD, then there should be a section in your user manual about how to reset the unit.