Before trying to unclip the tender top try to apply a 12V DC power supply to the plain wheels on one side of the tender (i.e. not the ones with rubber traction tyres) and the metal pin that connects the tender to loco. If the motor works then that's a good sign.
If no joy, then you need to remove the tender top next. On this older model, I am pretty sure there are no screws holding the tender body to the chassis. There are two lugs each side of the tender underframe (black plastic) that clip inside the tender upper body about 1/3 and 2/3 of the way along the length of tender. If you put a very fine sgrewdriver blade in the narrow gap where the underframe and body join (about midway along the body), you can prise the body out slightly, and it should then be easy to pull it upwards.