Whether you bought frozen lasagne or you made a tray of it yourself and stuck it in the freezer, lasagne should be fully thawed before being cooked or reheated. While you can place frozen lasagne directly in the oven, you have to adjust the time and temperature or you will likely wind up with only the top layers being cooked and the middle still frozen solid. Allowing the lasagne to come to room temperature first is easier and you just have to follow the original baking instructions.
Place the lasagne in the refrigerator to thaw overnight or at least eight hours. It is ready when you can easily cut through it with a knife.
Defrost in the microwave, but check constantly to make sure it doesn't start cooking in some parts while other parts remain frozen.
Place the lasagne in the oven and reheat or cook according to your recipe or as instructed on the packaging.
Rotate the lasagne tray periodically when defrosting in the microwave.
If the frozen lasagne is in a metal tray, always thaw in the refrigerator and never put it in the microwave as it can damage your microwave.