Answer to: Why doesn't my click wheel work after I changed the battery?Like Majesty said, you most likely did not seat the connector properly back in place. If it is seated properly, however, then you may have damaged the connector or ribbon cable when working on the unit.
Answer to: it got wet and now it won't start upIf it got wet and won't start up that most likely means that the main board is fried. If this is an iPod Touch there is no hard drive installed as it operates on solid state technology. Even if it did have a hard drive though, that would not prevent it from powering up. In order to fix this you would need to replace the main board, which would cost too much to be worth your time. The best idea is to buy a new iTouch.
Answer to: Can I switch hard drives from other mac laptops?Any SATA laptop hard drive would be compatible with your MacBook Pro.