I ran Software Updater this afternoon and went from 5.4.0-47 to 5.4.0-53. Now booting normally locks up before the splash screen, but booting either in recovery mode or from the previous kernel works. Any suggestions on how to correct this?
(Nothing appears in dmesg for the failed boots.)
Hi there 
Maybe the kernel signature changed. Try to disable secure boot inside BIOS
Secure boot is already disabled.
I would try Ubuntu Boot repair disk, it has saved me a few times in the past from a re-install.
A few times I have had kernel problems (twice WIFI with a Broadcom card) and was just forced to use the older kernel until the next update. It happens.
I am on 5.4.0-53-generic kernel but don't have any issue. Try removing 'quiet' from the kernel command line options when you see the grub menu. It may give clue to what is failing. Things to consider, are you having default layout/filesystem as installer suggests or have different setup say for example, filesystem is btrfs. Do you have special drivers needed? Those packages might have changed/broken with kernel upgrade. May be updating initrd will help?! When you are booted in to earlier kernel, try running following (assuming you are using -generic kernel),
sudo apt update && sudo apt full-upgrade
sudo update-initramfs -k 5.4.0-53-generic
Or, unless you want to know what exactly is happening, you can continue to use the working kernel version and wait out. Like @jymm mentioned, I too have faced such issues in the past and subsequent update would fix the issue.