What can be the cause of this ?
BATTERY IN THE REMOTE KEY HAS BEEN CHANGED

 

Have you tried another Fob (remote key)?
Did it work? If yes then Fob might be bad or come unprogrammed.
If no then car could be in sleep mode. (this happens after ~5 days with car armed). Open car with key in the door lock should refesh system and fob works.
Might try putting the battery in backwards into the fob. This "shocks" the fob into working again.
Might try disconnecting car battery for 30 min or so. (make sure you have your radio code if you have a porsche radio)
All of the above fails time to visit the dealer and have them check the system with the computer.