How do I get my machine to boot properly into 20.04, if I do nothing I get a blank screen, if I hit shift I go to revovery and can boot from there

Something weird has happened to my desktop. It will no longer boot properly. I am running Ubuntu 20.04.02 and Gnu Grub 2.04 and Kernel 5.4.0-77. When I boot I have to hit shift to take me into the recovery screen....otherwise it will never boot. I have tried using Kernel with "recovery mode" and going through the various options...but it seems ok and when I launch it does so....and then when I shut down and restart, I am back to square one. I guess there is some problem with Grub....but what?

Another piece of information: I attempted to install Ubuntu on this machine from a CD. It loaded the CD, no problem, for me to trial Ubuntu...and then as I went through the steps to install Ubuntu.. (I was expecting it to install over the existing version).....it told me that there was no operating system installed....but obviously there is as I could not write this otherwise. This to my mind points to something wrong with Grub or the boot loader. Where to from here?

Furthermore I do not have a boot partition....this is really weird. What do I have to do to correct this?

Well, I fount this and thought Aha! the answer to my question!

https://howtoubuntu.org/how-to-repair-restore-reinstall-grub-2-with-a-ubuntu-live-cd

Alas, it was not to be...

ubuntu@ubuntu:~$ sudo mount -t ext4 /dev/sda1 /mnt
mount: /mnt: /dev/sda1 already mounted or mount point busy.

ubuntu@ubuntu:~$ sudo mount --bind /dev /mnt/dev &&
 sudo mount --bind /dev/pts /mnt/dev/pts &&
 sudo mount --bind /proc /mnt/proc &&
 sudo mount --bind /sys /mnt/sys
mount: /mnt/dev: mount point does not exist.

ubuntu@ubuntu:~$  sudo chroot /mnt
chroot: failed to run command ‘/bin/bash’: No such file or directory

Interesting, I just had the same issue when connecting my computer via HDMI to my Samsung Smart TV. I experienced exactly the same behaviour. Ubuntu was actually booting, but I couldn't see it booting due to the black screen. Other people reported that Samsung TVs and my NUC computer have some problems with the HDMI handshake. My solution was to replace the HDMI-to-HDMI cable with a HDMI-to-Mini Display Port cable. An alternative could be to change the screen resolution at boot time.

I'm not sure if your problem is related to mine. However, I hope it will give you some ideas where to look further.