Mercedes CLK 320 keys locked in the trunk

I believe there is an access panel in the middle of the back seat. Pull down the arm rest and see if you can open the pass through panel. If you can, try to hook the inside trunk release.

I can't starting the car with the key

Replace the battery in the key. If it still does not work, you may have to take the key down to the dealer to reprogram it. IF you accidentally placed the key on a checkout counter that has a canceling magnet to disarm the security button in the product security system, it fried the commandos in your key. Some charge cards or even hotel keys can cancel themselves out. That is a common problem in hotels. If you have another key, try it. It it works, you need to reprogram the key if a new battery does not work.