Hi and welcome.
Assuming that you have indeed swapped units like for like, then the most obvious thing to check is that you've terminated your wires correctly. The transformer/timer unit has two lives going in - a permanent one and then a switched one (usually taken from a light or a pull cord), the neutral and earth. This is so that the fan can continue to run on the timer AFTER the light/pull/whatever has been turned off.
There's then +/- 12v out to the fan. Just take your time with a meter to identify which is which. If you've managed to accidentally connect the fan to 230V then it will in all likelihood be very dead.