Always have other yeast options on hand. Activity can be hard to see, especially if you used antifoam during the boil and are using a stirplate. It's likely that it fermented out if a two day cold crash in the fridge yields a clear beer. Gravity check would be the best bet. Is it possible that the yeast froze during the storage period? Domestic refrigerators can swing a lot. If you had extremely poor viability, and the starter did end up taking awhile to get going, you should make yet another starter from the sediment from the first.