OP, it is extremely unlikely that you could damage your finals by keying your mic even with voice or noise
present. All modern solid state transceivers have fold back circuitry that prevents enough power in the finals to blow them with an impedance mismatch.
What happens when you try to turn the set on? Anything at all? I suspect a fuse or a reverse polarity diode. Pull the fuses
and check them with a meter or continuity testrr. If they test good, take it to a shop. The diodes are a cheap
Another check... If you are feeling brave, or reckless, open up the unit and then connect your DC power, find where the DC comes in to the unit, and test for it inside the unit. Could be the power plug
or jack has a bent pin or something. Don't get zapped and don't conduct 12vdc anywhere it shouldn't go. Better yet take your power cable in to the shop with thevradio and let the repair tech check it out.