Alas, there are a number of issues that could be the problem, from something as simple as a switch, to the worst possible issue, the magnetron tube. However, it is not in the wheelhouse of the average DIYer to repair microwave ovens, and small appliance repair shops are getting harder to find since most folks don't want to spend half the of the cost of the device (or more) for repairs. This is why manufacturers offer warranties on these products, as they know once they fail, replacement is the most likely course of action.