Laptop ac adapter cord failure?

total weirdness tonight. My laptop died, so I presumed I forgot to plug it in. but it was fully connected. So I thought maybe the port was getting loose, like my last laptop, but it still fits snugly. So I thought ac adapter/ battery pack thingy was over heated and not working (it doesn't have a light to tell.) We switched the plug on the ac adapter/battery pack (the universal one, that plugs the pack into the socket) on a whim and everything started working. The cord wasn't bent, broken, or cut in any way, and all the ends seemed fine. Why would the cord itself just stop working?