I figured out a fix! I went to my VeraCrypt settings, switched on all the 'auto-dismount' options. Then I did a re-boot of my laptop. This time VC was triggered to do an auto-dismount. After that I could log back in as before.
I re-booted my computer without first dismounting a VeraCrypt volume. Now when I try to mount the volume it says 'Warning: The host file/device is already in use!'... And warns me that if I continue mounting this can cause 'system instability'. (No other people are using the volume) I am worried that if I proceed it may corrupt the volume. What should I do? I'm a new VeraCrypt user, so any advice would be gratefully received.