I will chime in here with respect to ground.
While one can measure voltage at the amp with reference to ground, that ground can often be through any number of tiny wires, or circuit traces back to the engine block and/or the battery. You can have a fully wired stereo, remove the ground powre wire for the amp, and still measure +12v at the amp's power terminal. The ground in that instance is usually though the RCA cables, and then through the electronic circuitry inside the head unit; certainly not enough current capacity there to run an amp or even get it turned on.
While we can check every power connection and every fuse to make sure that there is a solid path for high-current power from the battery, we similarly need a solid high-current ground path back to the battery. Make sure your ground wire from the amp to the battery is solid, without bad connections.
Now, you still share that your head unit is not working. This and the fact that the amp is not working points to problems elsewhere. What is the one thing that you might have done prior to winter layup that you have not undone yet? Battery switch? Do you have dual batteries? Did you conect grounds for both? Where does the head unit get its power?
Do some more checking.... Check the complete circuit path, powre and ground for both the H/U and the amp, then get back with us...
You'll figure it out!