I changed the SMART Key Battery and now can't lock and unlock car with my hand.

Hello, There are a few things to consider, and in the end, all roads may lead to a repair shop with STAR diagnostic software to find the culprit. If the key will start the vehicle, and the touch un/lock works on the passenger side, this is likely an issue with the door handle. However, if neither side works, the owners manual will describe how to un/lock the doors with the metal key. If that does not resolve your problem, use the smart key instead of your finger to press the start/stop button for the engine. Afterwards, turn it off, exit, lock with smart key button, and immediately try to unlock by touch the handle. If these do not help, you'll need to ask which shops have STAR diagnostics software: http://repairpal.com/mercedes-benz-within-10-miles-of-sanford-michigan-48657