my first thought too..gfi on a receptical..check all and report back
x2, it sounds like some of what you have isn't done correctly and to try and diagnose it like this is going to be hard to do.sounds like a loose nutral but you may want to call a professional
^^^^^ what he saidHow did you check for power coming into the switch box. If you tested it from the "hot"/black wire to the ground/bare wire it could be you have "lost" your neutral (white wire). Check the power by reading from the "hot" wire to the neutral and make sure you still get a reading.
OK, forget everything above, sounds like you fucked up your travelers on the 3 way. To fix it you will need to identify the wires and where they go. You need to try to make sure you hooked the wires back up exactly where they were, one of the screws on the 3-way switch should be a different color and the wire that was hooked to that screw on the old switch has to be on the different colored screw on the new switch. Also the white wire should not be a ground, it is a neutral and should not be "grounded" to the switch.
. . . the switch you bought is a 3-way right, three screws total on the side of the switch, two on one end and one on the other and then a small green one on the metal frame of the switch ? You are only mentioning two screws.Tell me if this is right or makes sense.With switch by back door in the down pos. the switch in the garage has power on the bottom side top doesn't until I flip garage switch then both top and bottom are hot.Well if I flip the switch inside the house by the back door the garage switch is the exact opposite then.