Unable to boot Ubuntu because of some kernel problem

My PC was working fine until yesterday. Today, when I turned on the PC, the OS didn’t even boot. I waited for 2-3 minutes but nothing happened. I pressed the restart button. Now the GRUB menu appeared and I chose the Ubuntu option. The OS booted and the Ubuntu logo appeared but the lock screen didn’t appear. It was just loading. I waited for couple of minutes. Nothing happened. I also tried REISUB, but it didn’t work. I pressed restart button once again. The GRUB menu appeared and I chose Ubuntu. This time I got this error:

Kernel error

I use only Ubuntu (not dual boot). My OS is Ubuntu 20.04. I didn’t tinker with any important system files yesterday.

What should I do to fix my PC?