Help.... my tortoise is missing..... how to save its life.

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Tortoises do go walkabout. And, worse still, they are easily stolen or picked up to be "rescued" by passersby who take them home. They can burrow under fences or  break through them like small tanks. Even tortoises kept in an indoor enclosure may find their way out and hide under a piece of furniture.
Sarah Joiner in central London lost her tortoise Zuma, aged 90 earlier this week. Luckily she guessed that he might have crawled into a black bin liner with the rubbish. She phoned Westminster Council who agreed to help.
Zuma was on his way to an incinerator, but satellite tracking by the council located the lorry in which he was travelling. After sear hing through one thousand black  bin bags for more than two hours, Sarah noticed some flowers she had thrown away poking out of one of the bags. Inside it was Zuma "on his back , legs flailing, looking incredibly indignant" she said.
Lottie a tortoise went missing for two years. Unlike the long journey of Zuma, she was found only a little way away. She escaped out of the garden into the neighbouring playing fields and was found in the far side. She had successfully survived and hibernated there. Because she was microchipped, her original owners got her back. 
So what to do if your tortoise is missing? A careful search in the garden, looking particularly for any fresh earth or in spaces like under the shed, may turn him up. Check the fencing for signs of burrowing or pushing through. (Remember that tortoises need opaque barriers. They will continue to push at glass or netting.)
And how to prevent it happening again? Check boundaries in the garden daily. A fence should be 8-12 inches below ground and high enough to prevent climbing - yes they can climb and will chimney up corners like a rock climber. Lock up your tortoises at night, just like you would your hens. They are like a meat pie with a hard crust for foxes and badgers. And dogs will attack them too, even the family dog.
Microchip your tortoise (yes, it is possible). And don't have a garden pond. Tortoises can drown in a pond. They can't swim. If you must have one, fence it off and cover it over. More vital information on how to protect your tortoise is here.
And please do my Reptile Relationship survey here.