Hi and welcome to FixYa, I am Kelly.
If you are sure that you have your electrical connections correct, that the exhaust fan is not running and this is a NEW unit you need to go to this link and first Register your microwave:
You have a product warranty claim and should not attempt any repairs. You will void your warranty if you do anything else.
After Registration making sure you state it is a warranty repair you should schedule warranty service here:
GE Warranty Management
Monday - Friday: 8 a.m. - 8 p.m. ET
Saturday: 8 a.m. - 4 p.m. ET
My from the hip thoughts are that there is a control board, or relay problem in your microwave. It could be several things including a loose wire... but at this point your stuck in the warranty repair routine. Anything I would tell you to do today would void your warranty.
If you need more assistance just respond here and I will help you.
Thanks for choosing FixYa,