My 2003 1100 just lost all power. no horn lights or start. checked fuse box n battery. still nothing. ANY HIDDEN FUSES?

First use a socket to turn crankshaft pulley. Prove engine will rotate. If it does rotate; your engine isn't locked. Go to fuse box under hood, check fuses. Google how to use test light if you don't know how. They're cheap to purchase. Identify crank relay in fuse box. Swap it with identical one ie horn or ac relay. Oh! Check battery connection first. Should be tight. Turn on headlights? No have battery checked.

No headlights, horn, or brake lights? I would expect nothing at the ignition switch, either, then? No warning lights or gauges with key on?

Any chance the battery is discharged? It would have to be very low or dead if connections are clean and tight. Inspect the cables, take them off and clean the terminals of corrosion, tighten them down good. Check the ground where it bolts to metal-if not a good ground, power will not flow. From the battery, power is sent through fuse links and maxi fuses in the power distribution box, to distribute power through all the car's circuits. Either a fuse link or a maxi fuse will send power to the ignition switch. Another will send power to the headlight switch, and another will send power to the fuse panel under the dash, powering those fuses that are hot at all times (like the brake lights and horn).
It sounds like your power is not getting from the battery to the power distribution box, if just nothing works. Bad battery or bad connection, possibly.

Could be, sounds like a big electrical short somewhere. First check that it didn't kill your battery. Have it tested. If it is old, just replace it- you have to have a fully charged battery to do any checking or testing. Check all your fuses, and it would really be good to know there is power to the fuse. All of the maxi fuses are hot at all times. Use a simple test light to check they have power-one will feed the ignition switch and one or two others are for the head lights and lighting. You can check every fuse on the car, both underhood and inside the car at those fuse panels -with key on EVERY fuse on the car should have power to it. Just touch the test light probe to the top end of the fuse, that little indent is a test port to check for power. Every fuse has those openings, one at each end. A good fuse will light your testlight at both ends of the fuse. A bad or blown fuse will only light one end of the test light.
You may have lost power right off the battery if nothing works. How warm were those battery terminals? A poor connection will create a lot of heat, but that would likely be hot hot...lol
Start with a fresh battery, make sure cables and connections are good, make sure the fuses are good and have power, then hit the switch-if it blows something, bad switch or a short in a wire.
Good luck, Don.

THERE IS A MAIN FUSE FOR ALL AND AT THE BATTERY BUT THE FUSE BOX DOES HAVE A PRIMARY FUSE FOR JUST THAT INCIDENT CHECK AGAIN GOOD LUCK

The horn and lights are not controlled by the key switch.
It sounds like the switch has failed or you have lost power to it from the battery.
Have you checked the fuses and the key switch with a meter ?

check the large fuses in the under hood fuse box if someone jumped it backwards it blows a lot of fuses out

Check all of the fuses located in the black box under the hood close to the battery on top of the inner fender. All power to the car begins there. Check for large fuses 30 amp to 60 amp. Make sure the heater fan is left unplugged. When working on components under the dash always disconnect the ground cable from the battery

Sometimes the fuses will look OK, but still be bad. Did you check the fuses with a test light?

You have to have battery hook up to check fuses with test light,
When you unhooked battery you put vehicle in thief mode,
thats why horn honked and disabled ignition.

you need to check battery supply most important. if the battery low voltage, cause no start. if the battery ok & strong energy. maybe the drive away protection control if this is the problem you need to bring to BMW dealer to replace control & ENCODE the control unit using SSS.