If your Android fails to boot after an update, don’t panic! Hold the power and volume down / volume up key (depending on phone) to boot into fastboot mode.
From here, you can flash an update that fixes the issue. If no such update is available, you likely have to wait for it. Now, on LineageOS recovery you can click on „Apply update“ and then select ADB. This allows you to use adb to „sideload“ the update file. Just download the latest image and type
adb sideload <filename>.
Don’t flash the update again! You will lose your other flash bank, too. This error doesn’t come from the system update but rather files that got corrupted.
From fastboot mode simply switch back to the alternate flash bank by typing
fastboot --set-active="a" if you have slot B active or
fastboot --set-active="b" if you have slot A active. Check the active slot („current_slot“) with
fastboot getvar all.
Chances are that your phone might boot again :0