30 - Eight Keys to Eden by Mark Clifton

When Earth suddenly loses contact with the colony planet Eden, a daring expedition is launched to uncover the truth. What they discover is beyond comprehension the colonists, once equipped with advanced technology, are now scattered and disoriented, wandering naked through the underbrush. Stripped of their tools and civilization, the expedition team is left to unravel the mystery of what happened to the lost colony. - Summary by Dale Grothmann