Lasagne package says cook from frozen
This issue has come up in our home. 
 We did an experiment with a frozen homemade and a frozen store bought lasagna.  We defrosted both of them for 3 days in the refridgerator.  We did this to make the process of heating them easier, as it takes a much longer time to heat them up frozen.  They were both approximately 10 pounds each and took an oven each.  We heated them covered up at 325 degrees for about 90 minutes, and the last 15 minutes cranked up the heat to 400 degrees uncovered to give it a bit of a crust and also added more fresh cheese on the top. 

Under no circumstances freeze it again just to cook it frozen, because it just ruins the texture of the cheese, the sauce gets watery and it's just not a good thing.  

If you have leftovers, eat it the next day or two.  Remember, all components of the lasagna have been cooked already (unless you made it fresh and used the dry cooked lasagna layers) but a store bought one, most likely has been cooked already. 

Now if you have smaller 1 or 3 pound ones, you can put them in the microwave and they will be just fine, and no, a few days defrosted is also okay, and most likely 5 to 15 minutes on high heat, rotating the lasagna will be all right.