Hi @Jeff Ferris
What is the model number of the microwave oven?
Have you been stopping the oven from operating by simply opening the door and not pausing the operation using the control panel, by any chance?
If so there may be a problem with one of the door interlock switch contacts remaining closed when it should be open, when the door is opened. With a lot of microwave ovens there are 3 separate door interlock switches. Two that close when the door is shut and one that opens when the door is shut. They are operated by the latch arms on the door.
Opening the door whilst the oven is still working, will stop the oven operating as it is a safety feature designed to prevent harmful microwave radiation from escaping out through the open door. (With the door closed the oven becomes a Faraday cage).
However it is not a good habit to get into. Doing this will cause the switch contacts to arc and this eventually causes contact migration which means that the switch contacts become permanently welded together instead of opening and closing.
If you have only ever opened the door after the oven has stopped operating or have paused it using the control panel, then it may just be a loose switch not correctly operating and releasing.
To find replacement switches (if one is faulty) search online for (insert make and model number of microwave oven) parts to find suppliers that suit you best. Some have parts diagrams that show the layout of the microwave oven e.g. searspartsdirect.com and partselect.com
If you decide to check if the interlock switches are OK etc, also see the safety advice in the answer above