1998 Mercury Sable car won't start

Ok. This has puzzled several mechanics.I drove my car to a store, parked it, shut it off. After coming back out, the car wouldn't start. When turning the key, there is no 'click' sound, no noise, nothing. After several failed attempts to jumpstart it, nothing. I left it there for 2 hours till a friend came over to help. All he did was wiggle the negative wire to the battery, and it started! How ever. After going back home, parking it, shutting it off, waiting several seconds, I tried to start it again, nothing, its dead.I let it sit overnight, and this morning, I went out with a new terminal connector, got in it, turned the key, it started. But again, after turning it off, it wouldn't start. I changed the terminal, got back in it, and its still dead. Im lost.