I know this is a bit late to reply on this thread, but I thought it might help someone else out there.
I recently had the same problem with my HP Pavillion dv5-1235dx. When I tried to boot up, it froze at the HP Invent splash black screen, with "Press ESC for Startup Menu" down in the lower left-hand corner of the screen. When I tried to press ESC, nothing happened, other than that message changed to "Press ESC...pause". None of the keys (or key combinations, including cntl-alt-del) would respond at that point. When I tried to use any of the f-1 through f-12 keys during the first couple of seconds, when trying repeatedly to start the laptop, it would show in that same lower left-hand corner a word or two about what that option was, but wouldn't respond at all beyond that. I couldn't reach the BIOS setup, boot order, or any other screen. I tried to put a Windows installation disk in, but even that wouldn't work. I could see and hear the disk being read, but nothing would happen on the screen, and the keyboard, touchpad, and external USB mouse all were completely unresponsive. Nothing that I read online in any of the forums worked. I tried removing the battery for several minutes; tried removing the CMOS battery; tried removing the HDD; and tried removing the DVD-RW drive. None of these attempts helped in the least.
Finally, two things clicked: One, I read several different posts where those with a similar problem mentioned having put the computer into hibernation just before the problem arose; and two, someone suggested that bad RAM might cause such a problem. After I thought a little about it, I remembered that the last time I'd used the computer, I'd tried to put it into hibernation, but simultaneously, the battery - very low on juice - gave out, and the computer shut itself off.
The solution: I removed ONE of the two RAM memory modules, and then tried to start the computer. It suddenly whirred to life, coming up with an error message, saying that all unsaved data might be lost, and asking if I wanted to continue. I clicked on "yes", and the computer then started up normally. Thinking the memory module was bad, but wanting to make sure, I shut the computer down, replaced the missing module, and restarted the computer. It started up just fine.
Apparently there can be a problem, under the right combination of circumstances, with the Hibernation option (making this problem, I would think, a Microsoft software problem - (my laptop runs Vista). When entering Hibernation, the computer is supposed to save a snapshot of the currently opened programs and windows to a file on the HDD. Then, when the computer is switched back on again, it should read that file back into the computer's RAM memory. I'm guessing that, because my laptop's battery just happened to go dead enough to cause the computer to shut down - IN THE MIDDLE OF WRITING THE HIBERNATION DATA FILE (before it was complete) - it caused the computer to hang on startup, as it tried to write that (unfinished) hibernation data file to the RAM modules. When I removed one of the modules ( I think they're supposed to be installed in PAIRS), it finally broke the hang/loop and brought up the error message. There was nothing actually wrong with the memory module itself.
I hope this helps someone in the future. I'll pass this along to Microsoft, and see what they think of it.