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I think the first thing to try is, remove any SD card or other memory card that might be in the device's slot, and then try powering the device On again normally.
If that produces no response, disconnect the power (or, the + battery lead) for 10 minutes +, then try the device again.
And lastly, if that still gets nothing to happen, then try holding down Power, +, and - all three at once for at least 10 seconds continuously.
Then try pressing and holding the Zoom + and - buttons simultaneously, then press and release the Power button, and continue holding the Zoom + and - buttons for 30 more seconds.
As for wiring color-coding standard... it all depends upon who is actually doing those standards, and whether they care or not.
It is quite possible for hot to be color-coded black, as misleading and dangerous as that can be. Yes, I have heard of it, even in household wiring... a place where such sloppiness is dangerous.
Does the Lowrance power up now? that's the proof of the quality of the wiring feeding it ultimately.