Its because you got the wires the wrong way around. It doesn't necessarily mean the board is gone or the fuse is dead (the amount of times I've convinced myself I've blown a board, only to find it working a day later is astounding. The board seems to have some mystical power that makes it heal itself or something)
Check the fuse with a multimeter. If you get 1 across the fuse, then it has blown. You can replace it with fuse wire if you don't want to bridge it. If it hasn't blown, then you're probably safe. Just disconnect everything and leave it for a few hours. Wire it back up with the correct polarity this time and then give it a shot from the mains (first with the fan only, then with the fan and the board, and if still nothing, the fan only by itself)
If the board still doesn't work, and the fan doesn't work afterwards, you have a short somewhere or incorrect polarity still which is tripping your mains plug. This is also magical and heals itself when you unplug it from the mains for a few minutes.
If the fan doesn't work the first time, try another mains adapter.
Let me know your results.