If the amp is in protect mode, it probably has shorted output transistors. To eliminate other possible problems, do the following. I'll assume that you've checked the amplifier's on-board fuses.

With your multimeter set to DC volts, the black meter lead on the ground terminal of the amp (not on the point where the ground wire connected to the vehicle) and the head unit on (so the amp will have remote voltage applied), touch the red lead alternately to the B+ and remote terminals of the amp. If the voltage is below ~11 volts, you need to check the wiring feeding whichever line is too low.

If the voltage remains near or above 12v, disconnect all speaker wires from the speaker terminals of the amp and disconnect signal cables from the amp. If it powers up, the wiring needs to be checked. If it still shuts down, the amp almost certainly has shorted output transistors and will need to be repaired.

Let me know if you have any other questions.