This issue comes up very often regarding these systems. "Protect" is an indicator that there are either shorted speaker outputs inside the unit, a shorted speaker wire, or a blown speaker connected to the unit.
If you've pressed the reset button, and it's still in "Protect", then disconnect all of the speaker wires connected to the head unit. Isolate the wires on the Kenwood from touching each other and power up the unit again. If it starts normally, then there is a shorted speaker or speaker wire in the vehicle somewhere.
Start connecting the speakers one at a time (shutting the power down before connecting each one), until one of the speakers causes the "Protect" indicator to come on. This speaker is faulty and would need to be replaced.
If the unit shows "Protect" with NO speakers connected, then it needs the output transistors replaced inside the unit. They are not user-replaceable as they are PCB mounted, and thermally coupled to a heatsink. In this case, the unit is in need of service.