The best case is if the plug/cord stopped working. First, get a voltmeter, plug in the cord and check to see if you get a DC voltage on the plug that goes into the computer. If that has DC power, then the problem is inside your laptop. If you don't have power, disconnect the plug from the transformer and see if you have an AC voltage on the plug. You can keep walking back until you find the portion of the wiring that is bad.
My first bet is that the problem is internal to the laptop. Those parts are much more sensitive than the cord. Second bet is the plug that goes into the computer, third is the transformer, but that is unlikely.