disconnect your battery first.
disconnect all of your amp, speaker, and head unit wiring to eliminate any possible shorts.
next, check all of your fuses located both in the kick panel, under the hood, and on the amp and head unit.
replace any blown fuses, then reconnect the battery and check for power to the car.
if no power, see mechanic or ask in a mechanics forum, if power, reconnect head unit and check it for power.
if hu has power, good. count your blessings.
next, apply amp power and ground to check for function ( make sure your disconnecting the battery between these steps)
if you need to, use about 6" of 20+ gauge wire from amp power to hu turnon input.
if amp has power, awesome! if not, can't win em all.
to test the speakers, use one of those multimeters on ohm setting. if it reads either zero ohms, or infinite ohms, theyare blown.
if it reads standard ohms for the type of speaker, lucky you!
.
next step is to replace what's broken, but new wires and inline fuses, and never connect an amps output wires to anything other than a straight connection to the speakers.
but. to explain what happened, you must likely completed a circuit from battery, to amp, to wires, to head unit, through ground, then back to battery.
and thus, cooked wires, blown fuses, and a lot of trouble.