I think my battery is flat, I cannot gain access using my key fob?

When the ignition key is removed, loads which are switched on or on standby (e.g. power windows, luggage compartment illumination, interior lights, radio etc) are automatically switched off after approx 2 hours.

If the vehicle is not started or unlocked with the remote control/key fob within 5 DAYS (or 7 days on a 987/997), the remote control standby function is switched off to save the vehicle’s battery.

To rectify this carry out the following steps.

Unlock the driver’s door with the key at the door lock, and leave the door closed in order to prevent the alarm from being triggered.
Press button 1 on the key fob.

The remote control is now activated again.

Also see this Gaining Access To Luggage Compartment