1) Confirm battery is good through some type of concrete means, if not already done.
2) Make sure all connections are tight; terminals to battery, cables to terminals, ect.
If both these appear to be true, as a double check, you can look for power at fuses both inside the cabin and under the hood. If you find nothing, it has to be either a cabling issue (possibly a blown fuse link) or a blown 'maxi fuse' or other large circuit protection device.
Does ANYTHING work?
No power anywhere
If you have no power to the horn or lights and such then you should check the battery terminals, the battery cables, and the ground. It could also be a really dead battery but that is really dead.