Do be careful poking around inside a microwave. Electrically speaking, they are arguably the most dangerous common appliance to work on; the magnetron supply produces a couple thousand volts at a significant current level (maybe a half an amp), a potentially lethal combination, and can store a charge even when power is removed. Usually they are designed with bleeder resistors to dissipate that charge, but if the oven may be broken it's no guarantee that that isn't a broken part.
One common cause of a blown fuse in a microwave is a dirty or misaligned door switch (which forms part of a safety interlock system). There are generally two sets of contacts for the door switch: one, which operates first, signals the control board to shut off the microwave. The second set acts to cause the fuse to blow if the magnetron is energized in case the first set or the control board failed to shut off the microwaves. (Another common cause is a failure of the high voltage power supply or the magnetron. Very occasionally it may be due simply to age and nothing more.)
