1994 GMC Seirra wont start tried changing the battery and still wont start,when we changed the battery the dash lights and the headlights worked but as soon as we turned the key all lights went out and the truck would not start the lights will not come back on.It will not even cranck or anything.

Try going back to basics. Start at the actual light and work backwards. Check the bulbs. Check the fuse. Check the earth points bolted to the body work behind the headlight are clean and shiny. Check the relay in the fuse box.

try replacing the bulbs. They don't last forever.

See fuse for the headlight low beam or the head light low beam filament is bad change it after testing it on the battery

which lights work? headlights? tailamps? dash lights? radio lights? Interior lamps?
turn suignals? flashers? Brake lights?

To find the FLASHER on 1994 GMC Truck C1500 1/2ton Sub 2WD 5.7L TBI 8cyl. the Flasher is located: Front seating area, driver side, under dash, driver side of steering column, mounted in relay bracket

Application: Hazard flasher

Turn SIgnal flasher is one independent unit.

Hope helps-

When you say you have power, do you mean the dash lights come on?
The power requirements of the starter motor are a lot higher than that of the dash lights or even the headlights and accessories.

Turn the headlights on and try to start the engine, if the headlights go really dim or right out then your battery doesn't have the capacity that the starter needs to turn the engine over and you need to replace it.

try car to car boost-battery charger may not be working correctly or set on low amp charge -interior lights and dash may all light but battery may not be strong enough to energize starter have battery tested if battery good than check for power to the starter if power-- starter may be shot can remove also and have tested at most auto stores free if no power battery -wiring-or connection issue would need to test fuse panel-ignition switch and starter solenoid

Headlight relay, perhaps? Should be in one of the fuse/relay boxes under the hood.

Check that Battery Cables are tight, Check Battery ground to Metal.Do headlights light? I am not aware of any 1 main fuse that would prevent lights, starter from working.Good Luck.

check your fusible links from the battery