The Velvet Drives that I have torn apart didn't have any shear pin inside. Inspect the coupler between the shaft and the transmission carefully. Depending on your setup and transmission you may not even be able to see the transmission shaft as it could be entirely covered by the coupler.
The trannys that I have torn apart were all direct drive units, and I remember them as having the coupler bolted to the end of a splined shaft. Unless the coupler just plain broke then there isn't anything that could shear.
It is also possible that your coupler isn't on a splined shaft but rather a keyed shaft. It is possible that the the "key" that pins the coupler to the transmission shaft has sheared off, which is like having a shear pin but it wasn't intended to work that way. If it has sheared, you might get away with just replacing the pin, but it might have really messed up the coupler and/or transmission shaft.
When the engine is running, do you hear any sound from the transmission when you shift from forward to reverse? If the transmission was still coupled to the engine then I would expect to hear some gear whine, especially when you shift to reverse. If there is no noticable noise, then I would concur with Peter regarding the coupler. I have broken flex plates before so I know it can happen.
If you get any reaction from the transmission at all, and you are sure the the output shaft is not spinning then something dramatic has happened inside the transmission. I have rebuilt a couple of these, and I can't imagin what it could be. I can imagin losing forward or reverse, but not both at the same time.
If you are a "do it yourself" kind of guy then I would recommend that you start by pulling the shaft and removing the coupling from the transmission. Completely, not just separating the coupling.
If the problem turns out to be the flex plate (engine to transmission connection) then replace it and assume that it was a faulty flex plate.
If the problem was with the shaft to transmission coupler then check the shaft alignment carefully when you put things back. If things don't line up well then the constant flex and strain will eventually break something.
If something inside the transmission broke then I don't know what to advise. I would expect clutches to burn out long before something broke.