Reheating well-pepared food that has been stored sensibly, in a fridge or freezer, for not too long, is perfectly fine. In fact, I make meaty stews and ragus with the express intention of reheating them, as they taste much better after a day or two or more, and we save meat from roasts for days. The risks are:

...you kept the food for too long, maybe more than four or five days for cold meat from your roast for example.

...you kept the food too warm for too long, for example in a warm kitchen.

...you reheated food that had been badly stored, as above, but not to a high enough temperature to kill germs.

Thing is, reheating food full of germs takes a few minutes, not enough for germs to multiply much. Reheat it to a high enough temperature for long enough and you're fine.