The only way to revert a system if you don't have the password is to reinstall using a backup. In fact, even if you have the password that is the only practical way to go backwards. Before you try that there are some things worth trying:
The best guess hope is that the keyboard on your boot screen is somehow messed up. In this case probably the easiest way is to
- Make a new bootable USB stick
- Boot into live system mode (do not attempt to install)
- Attempt to mount the drive from your system
- Enter the password you know
- Assuming this works, now change to an encryption password with only standard lower case letters
- Reboot and use the new encryption password which should work on most keyboard layouts (except ones like the German ones with letters in different locations)
In the case that the password still doesn't work in live system mode then the most likely explanation is that your old system had a different layout during boot. In this case, you can experiment with setting different keyboard layouts in the live system and entering your password as you would have normally. Definitely try EN-US and EN-UK but also look at the ones that were next to your normal keyboard in the list during installation.
If you have really valuable data that wasn't backed up, unfortunately it is likely lost, however you can consider taking a low level image of the whole disk or just replacing it and keeping it. This will allow you to try again later.