blown internal fuse or link
positive and negative touched and blew something inside
The fuse is inline with the power cable. Here is a link which shows you where it is.
If the fuse is blown and a replacement fuse (ensure that you replace with the correct rating fuse rating is usually stamped on silver cap of fuse e.g. 12V 2A etc) blows immediately you connect it, then there is probably a short circuit on the power input.
If you find that fixing the fuse didn't solve the problem and it doesn't power up, here is a wiring schematic which may assist you to find out what is wrong.
Here is a link detailing a solution by someone who had a similar problem http://www.radiomods.co.nz/forum/viewtop...