Easiest way just land in a farmers field walk to the Farmhouse and ask where you are.
Once he has put you right jump back in, takeoff and away you go....simple
But make sure you are insured for take off from an unlicensed airfield! Also if you are a student operating under the supervision of a flight instructor the authorisation ceases once you have landed. You would need to contact your FTO/instructor for recovery and would not be authorised to take off.
The other point worth mentioning is why pilots get lost. Ok it can happen to anyone but thorough pre flight planning and good training will go a long way to avoid this happening. Before you use GPS (or any other radio aid come to that) you need to understand the basics of pilot navigation.
That said if you really are lost then in the congested airspace of Europe you need to talk on the radio asap!