This is a common problem with the 850's. Mine quit too and I had to replace the little gear inside. I'm 99% sure that yours is the same problem. Although there are probably cheaper places to get this gear, but the following comes with a DVD to explain the procedure for replacement if you're up to it. http://www.ipdusa.com/ProductsCat.aspx?CategoryID=1732&NodeID=5357&RootID=629
The following is without the instructional video, and cheaper:
http://www.odometergears.com/subpages/volvo.html
You can probably find them on eBay too.

If you are not up to doing it yourself, then I suggest a good indy repair facility or the dealer. You can take the instrument cluster out, or have your indy repair shop do this for you and send the whole unit off for repair to this place: http://www.speedometer.com/ (That's what I did several years ago before these gears were available on the web, and the cost was about $100... I was without odometer/speedometer for several days.) However, if you're going to go as far as taking out the instrument cluster yourself (you do have to take the driver's side of the dash loose from the car), you're about 90% there in regards to fixing the problem, so why not go ahead and do it yourself. Just my opinion.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon