Is this the one with the reset knob on the back, or the push button on the front?
If it's on the front, sometimes they hang up, and the mechanism doesn't return so the gears again mesh.
If it's on the rear, it's probably like all the rest of the airheads that reset that way. And in that case, there's a gear that gets loose and then jams, or allows all the other number wheels to flop around. Bad stuff can happen - and this may well be happening with the style speedo with the button on the lens.
The best thing is to disconnect the speedo to prevent worse damage ocurring. It may only be the reset not returning - but I can't tell just by these symptoms.
Oops, after re-reading your post, I'd say the worst is happening because the odo also isn't working. I'm not positive, but think the odo still works when the trip meter hangs up.... I'll have to check that out.