It probably wasn't the temp change, you most likely dropped it in the cup or it hit the side. It doesn't take much. They are very fragile pieces of glass. HS kids used to break 3-4 syrup hydrometers in a season doing just that. They broke the sap hydrometer every year but that didn't get replaced. I think the el cheapo refractory hydrometers are like 20 bucks on amazon. For sap, I would use one of those, and maybe just check calibration with the sap hydrometer.