Hello,
Unfortunately, with so many possibilities it is impossible to say. Everything I suggest would be guesswork.
A diagnostic scan can reveal the actual operation of the vehicle, and can help guide your technician to appropriate diagnostic testing.
Especially with Range Rover, like other luxury brands, this will likely save money over buying parts in a hope that they fix the issue.
Check all of your fuses, including under the hood, and try using your spare key to start the engine. Also, listen for the fuel pump to prime (whirring noise from back of truck) when you open the door or turn the key to 'ON'. Look for obvious damage around the engine bay, and scan the vehicle for trouble codes with a cheap scanner.
If you have a security warning, this is likely the issue, if not, the fault will need to be flushed out via diagnostics.
Your best bet, unless you find a bad fuse or the spare key works, is to seek help from a professional, especially one familiar with Range Rover. Ask around these certified shops:
http://repairpal.com/land+rover-within-10-miles-of-lexington-north-carolina-27292
