I had same problem a few days ago. I decided to use my own skills to troubleshoot this and determine if it would worth repairing or if I should get a new unit. My microwave is only 2 years old. After I found a solution I thought it would be a good idea to post what i did to fix this...or rather work around this problem.
What I discovered was that it is the switch tab which was broken. for the Vent fan and the light switches. The problem manifested over a period of few weeks when i had to apply increasing pressure to activate these switches and finally one day they broke and went all the way in and i could not reach it.
I downloaded a Service manual online and got instructions on how to access the inside of the switch compartment. ( i'll try to attach pictures).
Basically what i found ( as stated by earlier posts) that the flimsy plastic tab broke. I figured gluing this or buying a replacement unit will only solve my problem for a few days as this is likely to break again. Basically there is no support at the back of the circuit board. so if you push had you are likely to break whatever is holding these switches in place. Just bad design.
So i removed those buttons completely. Added a thick padding using some non conduction packing material, so that any amount of pushing will not mode the circuit board from its place. The base where the actual circuit board has the round push button which is accessible from the outside. I can push a small plastic stick and operate these. So it's not pretty but it works fine