I have a '96 850 Turbo and sometimes I have the same problem. I found an article on brickboard last year when it first came up. It always happens when it's really cold outside.

I'm told it's something to do with the PNP switch in the shifter. A fix is to put the key in either accessory I or II, press on the brake pedal, and rapidly rachet the shifter from P to L a bunch of times (like 20 -50, really untill it works).

So you might try that, or find an answer from someone who knows more about it than I do.