No. Cooking or reheating food in a household microwave oven does not make it radioactive. Absorbed microwave energy becomes heat.
The FDA explains that microwaves are non-ionising radiation and heat food through their interaction with water molecules. Health Canada independently confirms that they do not make food radioactive and do not remain in it after the oven is switched off.
This does not mean microwave cooking has no hazards. The FDA describes risks such as burns and uneven heating. Those are separate issues from radioactivity: food remaining hot after cooking does not mean it contains trapped microwaves.
Microwave cooking leaves food hot, not radioactive. Heat and radioactivity are different things.
U.S. Food and Drug Administration
The FDA identifies microwaves as non-ionising radiation and states that their energy is converted to heat without making food radioactive.
Based on: U.S. Food and Drug Administration
Health Canada
Health Canada states that microwave ovens do not make food radioactive and that microwaves disappear when the oven is switched off.
Based on: Health Canada
U.S. Food and Drug Administration
The FDA discusses burns and uneven heating as microwave-related hazards. Rejecting the radioactivity claim does not remove these ordinary cooking risks.
Based on: U.S. Food and Drug Administration
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No. Freezing can stop bacterial growth without killing all the bacteria. Survivors may multiply again during thawing if conditions allow.