Some simple trouble shooting to help you identify the issue.
Firstly , begin by disconecting your speakers all together.
Turn the head unit back on, do you still get the "protect" message.
If it dosen't , then you know there is an issue with your speaker connection.
Otherwise, it is an issue with your power/earth wiring.
If it is your speaker wiring causing the issue, then connect each speaker one at a time, until you find the speaker responsible for the problem. Once you've found the speaker causing the issue, trace the wiring back to the speaker itself, checking for any loose ends, exposed wiring ect. Better yet, just re-wire that speaker if possible.
Be sure to have your battery disconnected each time you pull your head unit out. It dosent matter so much when connecting speakers so long as the head unit is switched off, but it's advisable that you do so, as a safety pre-caution.
If your speaker wires aren't the cause of the fault, check that your power wires are correctly crimped, soldered (which ever your method) and that your earth wire has a secure connection.
Also check that you haven't knocked a wire loose in the ISO plug (the plug at the back of the head unit ) especially if you've been removing, and re-fitting your head unit quite a bit.