I am having problems with my Maytag microwave MMV1153AAWMaytag microwave MMV1153AAW. Everything seems to be working except it does not heat up the food. Light goes on, plate rotes, timer works, but at the end the food is not even warm. can you tell me what's wrong

It sounds like the magnetron may have either failed or is not being supplied with the extremely high voltage required to run it.

YOUR SAFETY IS PARAMOUNT

This is a job for a professional but if you are safety clued up, here's instruction.

If it is the type with an "Invertor", they are not user serviceable and should be directed to an authorised repair agent for continued safety, otherwise if conventional;

Make sure the !!!!any capacitors are discharged!!!!! before attempting any sort of repair.
Check the door interlock switches first then the high voltage diode with either an AVO model 8 or moving coil meter on high resistance range for short circuit, (DVM's won't show the fault!), the capacitor can fail and go short circuit, the feed fuse on the primary of the high voltage transformer and then finally, the magnetron is best checked by substitution.
Hope that helps.