It sounds like the passenger side latch is not disengaging -- same thing happened to me recently.
Open the trunk and look up in the top corner on that side of the car. There is a small opening into the soft top compartment. Reach in with a coat hanger and try to find a black cable -- it's a bit tricky to locate but you should be able to find it. That cable connects the handle to the latch mechanism. When you find it, give it a tug and it should disengage the latch.
Since learning the hard way, I have attached zip ties around that cable on each side to make them easier to pull from the trunk the next time my latch gets stuck.
Good luck
Ken G
1971 280 SL
Silver/red