Hold them in place with string or electrical tape. Filament could be open — check with ohmmeter. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
This and the other parts are easily replaceable. With this design, the filament can be heated good and hot for long life and not have the tube run away. Thus, all the even numbered vanes are shorted to each other and all the odd numbered vanes are shorted to each other. Of course, all the rings are also all shorted at the outside where they are joined to the inner wall of the cylinder. This may cause expensive damage to the magnetron and other components. This will result in overheating as well as no or erratic operation. Most of these problems are covered in more detail elsewhere in this document.
Magnetron construction and operation The cavity magnetron was invented by the British before World War II. It is considered by many to be the invention most critical to the Allied victory in Europe. Unlike other consumer electronics where a new model is introduced every 20 minutes — some even have useful improvements — the microwave oven has not changed substantially in the last 20 years. Most metal objects should be excluded from a microwave oven as any sharp edges (areas of high electric field gradient) may create sparking or arcing which at the very least is a fire hazard.