Let an amp tech check this for you if you aren't sure. If you aren't sure, you can buy a tester that plugs into a three-prong outlet at any large hardware store or Radio Shack. If you MUST use a three prong adapter, you MUST ground the third prong. If you find a vintage amp that you absolutely must have, please make sure it has a grounded plug.
Strain reliefs are absolutely essential.
If the cord has slipped out of its strain relief, there is a high probability that one or more of the wires is broken or damaged. You will not notice this if the ground wire is the one that has worked loose. If the insulation on the conductors has retracted to where they can touch, the result may be a spectacular shower of sparks, or a possibly fatal shock . The time to find this out is NOT when you are plugging your amp in blind behind an obstruction in a darkened club. Re-install or replace the plug.