First, check the trunk, pull down the cardboard cover to reveal the back seat, you should see the black and silver amp behind that cover. If they are there and the sub is plugged into the silver amp, then check the sub separately via another source. If you get sound out of the sub then your sub amp is likely blown.
Just because a stock HU is in the car doesnt necessarily mean it was the one the car came with and therefore may not be connected correctly, although I doubt this is the case, just keep that in mind.
Go to that step and see what you come up with, then we can go from there.