I too have lowrance unit which is the reverse. it is powering up by itself as opposed to not powering up,despite checking fuses/lines which are ok. If you are lucky enough to be under warranty, take it back. If unlucky, you too, like me have paid big bux for something that has perhaps lasted only a few years and should last a lot more, given what you perhaps paid for it.
Technical expert advise is that initially the unit will power up by itself but now has progressed to not powering up.You may have a problem with the keypad failing. the bubble type press pad. The wiring shorts and the unit jumps voltage across the keyboard which "tricks" the unit into thinking that you have pressed the power key. After a period of time the unit problem will just progress to not powering up at all. basically the unit has ulitmately failed with time. Short of perhaps definitely stating the unit hasnt been hit or dropped, i suggest if you are in Oz, contact lowrance service in Sydney who are happy to help. they have a possible solution whereby you send in the unit, and they check it.
Generally if not deemed repairable they will replace it with a new similar unit. the only catch is you are up for X (few hundred dollars). this might seem a bitter pill, but you are only charged about a third or half of what a new replacement unit would cost. they give you an updated current model similar to what you have. Im prepared to take that option as it is a cheaper alternative to starting again and paying top dollar. Ive never experienced a power up/down and short of yelling at my son thinking he was playing with the unit, i will have to eat pie and realise with these sort of temperamental gadgets they can fail in the un normal way. Hope it helps. Kahuna 430