If you leave a frozen pizza out overnight the pizza can grow bacteria and spoil.
Frozen pizza should not be left out of the freezer or refrigerator for longer than 2 hours.
If you want to thaw the pizza out before cooking it then should place the frozen pizza into the refrigerator overnight to thaw out.
Leaving frozen pizza out overnight is not a good thing as bacteria can grow rapidly on the frozen pizza as it thaws out.
That bacteria growth causes the pizza to go bad and if the pizza contains cheese and meats then the spoilage of the pizza is usually faster.
A frozen pizza is good for at least one hour after it completely reaches Room temperature which could be at least 2 hours after it comes out of the freezer.
So it's good for at least 3 hours after having left the freezer.
In brief, eating pizza that was left out through the night is a bad idea.
The USDA recommends throwing away any perishable food (including leftover pizza) that has stayed more than 2 hours under room temperature.
Eating pizza that was left out overnight can cause foodborne diseases.
How to tell if frozen pizza is no longer good?
If dry spots or discoloration have developed on the frozen pizza, freezer burn has begun to set in - this will not make the pizza unsafe to eat, but it will harm the texture and taste.
The first signs of bad pizza are a hard and dry texture, still safe but not too tasty.
A spoiled pizza may also give off a rancid odor and become moldy if left too long.
Pizza left on the counter can remain safe for a few hours at most.
Food that isn't kept below 40 degrees Fahrenheit can lead to an increased chance of being a source of foodborne illness.
Eating a leftover pie that's kept overnight on your counter isn't advised.