The answer to 'why is my sump pump frozen' is that your sump pump can freeze up during extremely cold weather. This happens when the water in the sump pit freezes, preventing the pump from being able to operate properly. Common signs of a frozen sump pump include the pump not turning on, the pump running continuously, or water backing up in the sump pit.
To fix a frozen sump pump, you'll first need to thaw out the frozen water in the pit. You can try pouring warm water into the pit to melt the ice. If that doesn't work, you may need to use a heat lamp or other heat source to fully thaw the ice. Once the ice is melted, you can try running the pump to see if it's working properly again. If the pump is still not functioning, you may need to replace it or call a professional plumber to inspect the system.