Can you cook a digorno's pizza after its been thawed for 1 day
Yes, it should be.

That is - if it has been thawed, and safely stored (refrigerated, or in safe temperatures in whatever way), then it should still be safe to eat.  If it has been left out on a counter, or in otherwise unsafe conditions for more than a few hours - then it is as unsafe as the conditions have made it, and it is your decision what you will risk.  But safety-wise, while being frozen is safe, not being frozen doesn't automatically make it unsafe - not any more than any other kind of food that might be defrosted before cooking.

You might notice some changes in texture, and you will have to alter your cooking times, if you're cooking from thawed instead of frozen.  Your baking time will be shorter, for one, since the pizza doesn't have to come up to temperature.  You might want a lower temp and slower cooking method, so it is easier to keep checking when it's done (I usually do 350 and keep checking, while some frozen ask for 400 and specific times).  It might be easy to over-bake, since the instructions are expecting the pizza to be thawing.  It might come out a little differently - if it was intended to be baked in the bare rack, but now must have a baking sheet underneath because it's floppy, or perhaps if it was self-rising, and spent a little of that rise in the thawing - but I would expect changes to be relatively minor, or directly related to the change in baking method (crisper/softer crust from baking sheet or moister/dryer cheese from temp and time change).

I believe many manufacturers include instructions not to thaw the pizza because the results are more consistent if the process is, it is one less variable for them to worry about - and so thawing the pizza means it is one more for you to worry about.

If it helps, I've done this - thawed and baked a frozen pizza (it was thawed so it could be divided and baked over a couple days, and so was 'fresh baked' each time instead of dried from reheating).  I didn't have any particular issues with it, beyond having to change the cooking method to one suited to a non-frozen pizza.  Then again, I planned to do this - so you might have more issues with a rising pizza or if you're pickier about the outcome being precise or predictable.