When your microwave becomes cold instead of hot, it could be due to a number of factors. Here are some possible causes and how to fix them:

1. Power supply issues: Sometimes a lack of power or a power outage can cause your microwave to not heat up or cool down. The solution is to make sure the plug is plugged in tightly and check the power socket to make sure there is a constant supply of electricity.

2. Broken fuse: When the microwave oven overheats, the fuse will be triggered internally to avoid a fire. A blown fuse will cause the microwave to not heat up, so the fuse will need to be replaced to get the microwave working again.

3. Microwave oven failure: Internal problems such as component problems and circuit board failures may cause the microwave oven to not heat and cool. It is necessary to contact professional technicians for repair.

4. Magnet problem: There are magnets inside the microwave oven to generate microwaves to trigger heating. If the magnet is damaged or misaligned, it may cause the microwave oven to not heat up. The magnets need to be replaced for the microwave to work properly.

In short, the problem of microwave not heating and cooling may be caused by many reasons. You can solve the problem by checking the socket power supply, replacing the fuse, adjusting the position of the magnet, etc. If none of the above methods work, you need to seek professional help for inspection and maintenance.