Temps? How is the cage setup? I see you're using Carefresh but does the snake have access to a basking area and is there at least 1 hide in the cage?
Do you handle the snake regurally? Regular handling sessions can often stress any snake out to the point where they refuse food. Some, of course, are more tolerant to these sessions than others, but there is always a fine line when a person overdoes something, and this could be it.
The animal is W.C. so depending on how long you've had it, the garter still may need time to adjust. If the cage is located where there's a lot of "foot traffic" try moving it a more quite area of the house.
One other thing you can try - rodents are not the only thing you can offer, and if my memory serves me right (since it's been 15 yrs. since I've had garters...) they aren't really a staple part of a garters diet. Mostly likely you found the animal near a body of water; perhaps a pond or river. Garters feed verociously on earthworms and frogs. They also will except small fish (I prefer Rosie Reds), newts, possibly frozen/thawed quail, and possibly crickets.
I see brumation has been asked, well, with help from a friend, I just took my Northern Pinesnake out of brumation, and he's now back to his usual summer temps; feeding, happy and back to hissing like a mad man. So it usually does work with colubrids when the buggers refuse food.
But before you jump the gun, and brumate, we must first know your husbantry, and if not, wait at least two weeks before you even try putting the animal in hybernation. You need to make sure its entire system is cleaned out. Any undigested food still left in the digestive tract could rot, thus leaving you with a potencially sick animal. Their matabolism will slow down quite a bit and with temps around in the 50's, your snake will not be able to digest its food at the normal rate, or at least not quick enough.
You should also determine if the garter has good weight, girth and if it is healthy. The brumation process should never be tempted with a weak or unhealthy colubrid.
G'Luck!