The whole "loss of electricity" thing is a VASTLY simplified explanation of the premise. From what I've read, not only has electricity stopped working, but something has shut down guns, internal combustion, etc., like in the novel "Magic Time" or S.M. Stirling's "The Change" series.
Even laying all that aside, very few people nowadays know how to make or maintain black-powder and similar guns. It would probably be easier to reverse-engineer crossbows than firearms.