If your microwave is not getting hot then it's likely the magnatron in the microwave not working or the switch that controls the magnatron is bad.
When the microwave is closed the switch to the magnatron is supposed to close and allow the circuit to the magnatron to send electricity through it so it will come on.
If the switch on the microwave door is not working then that could cause the microwave to not heat up at all.
Usually it's not worth fixing the old microwave because the parts and labor costs to repair it could cost as much or more than a new microwave.
I never repair microwaves at all especially those cheaper ones because they are not worth it.
I just set the old microwave outside for a scrapper to take and recycle.