I had exactly the same problem with my 50 Merc four stroke efi. RNP hit the nail on the head. This engine has always started and while fishing Melones a couple of weeks ago, it went dead. I pulled and checked all the fuses under the dash and the engine. As RNP stated, it was a wing nut on the battery that worked loose. I replaced with a nut on both terminals....problem solved. Something else that Gary Burns of Take It To The Limit Guide Service helped me with. As I was preparing the boat to go home, I suddenly realized that the engine had to be lifted up as I had no power to the trim. Gary showed me a small circular hole, about 3/4" in diameter on the right side of the stem looking from the rear. I stuck a screw driver into the hole, found the slot, and turned cc to release the hydraulic pressure and was then able to lift the engine up. It was new to me, but good information for us boat owners that fail to read the manual. With all of the replies on this, I hope one of them will work for you. Let us know what the problem was.