Have replacement PC, but ADE says not authorised.

I have a replacement PC (old one died). Downloaded ADE but it cannot run to my KOBO because it says I do not have permission!!

Followed on line advice & ejected KOBO, reinstalled ADE and reloaded KOBO, but the device is not seen by ADE

Cannot authorise this PC because email ID & Password are already registered but cannot use replacement PC because the "dead" PC is the one the programme appears to recognises!!

Am I going mad or is this just techno stupidity??? I am at my wits end with this Magic Roundabout