I had to change my microwave Monday it arced off around the door seal, DW thought I was BBQing outside, it was her and the microwave.
From what you have described it might be the arm bracket detached it self or was never connected.
I unplugged the microwave then removed the screws that hold the bracket around the microwave and slowly and carefully removed it out of the hole.
It should not be hard to repair.
I went to wal-mart and found a microwave close to the same size HB 0.9 cu. ft. It had some red on it so DW like the upgrade in color.
I had to find the right bits for the screws that hold the bracket arms to the microwave it's a star with a hole in it, then there are some feet that are on the bottom that need to come off.
Now on the new microwave you have to drill some holes for the feet to be screwed back in do not over tighten it will strip the hole. I found some new screws for securing the brackets to the microwave, I adjusted the brackets to fit the front of the microwave Sotheby's door opens correctly then I drilled some holes and put the new screws, then I put the microwave in the edge of the hole and ran the electric cord in the hole to the receptacle slid the microwave the rest of the way and the holes for the bracket matched up perfect.
I made sure all the tape was all taken off and it worked as advertised.