why wont my gmc start?

Clutch switch if a standard transmission or gear selector switch if an automatic transmission. Common is the ignition switch. Anti theft system active.

Need a touch more info. Does it turn over and no start? No crank from key? Security if active will crank start and die, if no starter from key, automatic/5er? Ok ill give you a flow chart. Power when key is active ( if yes) go to next step, if auto while holding key and foot on brake move selector from park through all ranges and see if starter engages, if yes check neutral switch drivers side under truck on transmission, auto only turn key on jump solenoid purple wire w top terminal starter will crank if power is present, 5er chock wheels and set e brake. Prior to attempting this damage or personal injury may result. Speed flow chart is the same except you will need to stick your head under dash on clutch master rod will be a white plastic loop over it w two wires unplug and jump terminals w same gauge wire. Attempt to start. If none of this works post back and ill get you wiring schematic for start system.