smart keys

Just changed the batteries in the key of my C-Class 180 (2005) as it would not unlock the car but was OK starting the car. Now its the other way around, it opens the doors but not working when put in the ignition, any one any ideas?:(

I have a Sept 2000 W220 with the normal three button remote key. I also have an identical looking three button key which I bought from a local none MB garage which is second hand, and I don't know which car it came from. Can it be re programmed to fit my car?