Ok. If you have checked the round connector and are sure it's on properly, then here is what is most likely the problem: It's the "little blue fuse" as mentioned above. Even if it is still there and hasn't been knocked off, it probably got touched or bumped loose. If it is still in place, touch it with a screwdriver or something. It most likely will then fall off. Don't worry, that's exactly what you want. Now here's how to get your signal back:
Get a Caig Circuit Writer pen. (I got one at Radio Shack). Follow the instructions on how to use it. Practice on a piece of paper. When you feel comfortable that you can get a small dot out of it, touch it where the blue fuse thing was. Just enough to connect the two dots that it joined. Now let it dry over night. Put your phone back together the next morning and you will have your 3G signal back! That's it. I was without service/searching for a month after dropping it in water.