Suspend it in a pan of water on the stove. NOT touching the bottom of the pan. Heat water and watch when it opens. Should open before water boils. Take the tensioner carefully over centre with a bar.
The tensioner is spring loaded. Put a bar in on top of the tensioner and lever the spring that looks like a small shock absorber upwards to send the tensioner over centre. It should move freely on it pivot. They have been known to seize however.