I cannot switch it on after replacing the batteries.

I assume that the Magic Trackpad you have is the generation before the current one. In that case it doesn't matter if you pressed the on off button before you took the batteries out because it resets once there is nothing powering it. You could try cleaning the contacts in the battery compartment (with a screw driver). Just rough up the contacts a little. There might be some battery acid or corrosion on the contacts. If it still doesn't work make sure you have good batteries and you put them in the correct way. If that doesn't work it's probably a hardware problem and you could either get a new one or take it to the Apple Store and they might be able to fix it. Best of luck!