How to recover from a failed Migration Assistant process on a new iMac?

Migration Crashed!!

I purchased a new 256GB iMac and tried Migrating from my OLD 1TB. Oooops, I though everything was in cloud and not on the actual machine. Needless to say it stopped in middle. Friend told me to delete the largest file off the migration screen on new iMac and when it finished then we could transfer all the files that I needed individually. When finished new computer had only sign in as guest and time on screen and would not let me do anything. I found a way into the new iMac "Option and Right Arrow". That allowed me to sign into apple. but then open another sign in window "Guest and password" I was going to "erase and reset" and re start Migration assistant after eliminating a bunch off the old system. But it will not allow me to do that or anything else. It wants the "Sign in I use to OPEN this computer" which I used "Option & Right Arrow". I am lost.

[Re-Titled by Moderator]

Original Title: Migration Crashed