Hi Dave.
One simple thing you could try is still to take the battery out for a minute. Put it back in and start it again.
If this doesn’t help (which I would guess) you could try to boot from the original xxx.img straight from the PC using fastboot, like
fastboot boot boot boot.img
or (more likely to be successfull)
fastboot boot system system.img
to see if boot.img or system.img are damaged somehow on your phone.
For closer information and about where to find the xxx.img files, search and read in the forum.
If this works, you can flash system.img or system.img.
Somehow the phone (Android) seems to “believe” that the system has been updated. Otherwise it wouldn’t optimise the apps.