This problem does occur on occasions.
Don't panic - the lens cannot go behind your eye - a thin, but strong, membrane, called the conjunctiva, lines the inside of your eyelids and curls back on itself to cover the white part of the eye. Lenses cannot pass through it. However, your contact lenses may slide under your eyelids or become displaced. If this occurs, try looking in the direction of the lens to get it to move back to the correct position. If you wear soft contact lenses, they will tend to center automatically on the cornea. If you wear rigid gas permeable lenses, you may need to manipulate a displaced lens through the eyelid.
Your Optician can have a look under your lids to try to find the lens, he may need to roll the lid backward to get a better view, but it does not hurt to do this!
Usually if the Optician has had a look and not been able to find the lens, it is because it is not there, either because it was not inserted into the eye in the first place, or it has become dislodged and fallen out with all the poking about that has taken place.