I have an 8 year old desktop system with dual boot into Ubuntu 19.10 (default) Windows 10, or Windows 7. I've not made any changes recently, but it has begun intermittently failing to boot into Ubuntu. I get a black screen, I don't see the grub menu or any error messages, and it sounds wrong - it normally starts noisy with fans blasting and disks running up, then quietens down after about 5 seconds as it starts to boot. When it doesn't boot the racket goes on, the disk lights flash repeatedly, then the disk activity seems to stop. I've left it for up to 15 minutes but there is still no activity. Hitting Del when I start it up has no effect. I have to hit reset maybe 6 or 7 times and eventually it boots. I assumed at first that it was a disk or boot sector problem, but I had a look for the boot logs in Ubuntu, and each failed boot had a log, so it was getting through grub and was trying to start Ubuntu. I couldn't find any errors in the logs that looked relevant - there were a lot of ACPI errors, but I've been seeing these for a long time and online searches seem to suggest they are not serious. Each failed boot log was much shorter the logs for the successful boots, but each stopped at a different point, with no errors being flagged, just normal looking activity, then suddenly the end of the log. The problem does seem to be happening more often over time, which I guess might indicate hardware, but I don't understand why I'm seeing a black screen instead of the grub menu at the start when it doesn't boot. All I can think of to do is to change the boot order to boot it into Windows as a default, and see if that helps, but if anyone has any other ideas, they would be much appreciated!
Your hard drive may be getting a bit old. Try running
fsck from a live session to disable bad blocks.
Well, as the failures were getting more frequent I did consider running an fsck, but my partner happened to remember having a similar problem, which went away when he replaced his CMOS battery. I tried doing the same, and the machine is now booting OK every time. That's over the last 48 hours - I can't guarantee it's a permanent fix but it looks like a good thing to try if you have the same issue.