I left fully cooked lasagna out all night
Throw this out. The general rule of thumb is that food that isn't otherwise preserved (through  large quantities of acid or sugar for example) must not be in the danger zone from 40-140 degrees Fahrenheit for more than 2 hours. In practice this is an oversimplification - see the incredible and incredibly detailed food safety section in Modernist Cuisine for the whole story. Overnight is way too long. It is quite possible for bacteria to have multiplied and secreted toxins which are heat stable. E. Coli and Staph. Aureus both do this, for example. Reheating will not make this safe. It isn't worth making yourself or your family horribly ill. Order pizza.