Whether you bought frozen lasagna or you made a tray of it yourself and stuck it in the freezer, lasagna should be fully thawed before being cooked or reheated. While you can place frozen lasagna 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 lasagna to come to room temperature first is easier and you just have to follow the original baking instructions.
Place the lasagna 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 lasagna in the oven and reheat or cook according to your recipe or as instructed on the packaging.
References and ResourcesDave's Fresh Pasta: Pasta & Ravioli
Mama Yolanda's Gourmet Lasagna: Frequently Asked Questions