That is certainly an interesting situation! Assuming the glass isn't broken, I would assume it's not destroyed, but I'm not clear on how to get it back to its working state.
Under normal conditions, the bottom of the thermometer is filled with liquid, whether that's alcohol or mercury. There's a reservoir there that holds enough to fill the thermometer from top to bottom at its maximum expansion / highest temperature reading. Somehow yours has gotten the liquid displaced from the bottom so now as it expands it's expanding down instead of up.
I'm not at all sure how to get the liquid back down into the reservoir; if you can manage that it should go back to working correctly. I assume it got into its current state by sitting in the sun and getting too hot; perhaps the opposite will help, e.g., putting it in the freezer overnight? Or else maybe try using a heat gun at the top of the thermometer to try and expand that end and drive the liquid back to the other end?
I don't know, but I'm dying to find out how this all turns out!