I have a 72 gmc pickup truck with no electric,I went to start my truck and everything went dead I checked all the fuses,no burnt wires,replaced ign switch,and cables are tight,and I noticed after all went dead that the plug that goes into the alt had came unplugged,so does that mean my ALT went bad. Can someone,anyone help me with this problem

Coming unplugged is not an Indicator of a Bad Alternator. If unplugged, it won't charge.
Plug it in, Charge Battery, measure voltage, try and start, measure running voltage, should increase.