Unexpected SPC Access Width error returning to default e

What’s happened: I have modified the fstab file. My aim was to mount an external hard disk permanently. I duly changed the username, but forgot to change the "sdb4" to sdb1" in the command line and pasted it to a terminal window. Now Ubuntu does not boot, I am on a choo-choo train or loop, and I have forgotten the root password. This afternoon I created a boot repair usb stick, but this does not work either. F6 key does not work and I cannot choose the "no mode set". I fear I might well have to delete the Ubuntu partition (I am on dual boot: windows 7 /Ubuntu 18.4) and try a fresh install, thus losing some data….