I had this happen recently. I don't know why it happened... my Proxmox server has been running for months, it just happened spontaneously.
I read a bunch of threads (including this one), and found NO good answers. I decided I was going to try the install disk, and if necessary try to figure out how to reinstall Proxmox without killing my configuration.
Well, isn't there an "Advanced Options" item on the boot menu. Pick that and then select "Rescue Boot". When I did that, my Proxmox server just booted up, no problem. I figured it was the USB install disk so I rebooted without it and it booted smoothly into Proxmox. I didn't even have to do anything, the Rescue Boot simply fixed the problem.
Your problem may not get fixed this way, but it's the first thing I would recommend you try.