What would cause this??????

Hello FABO boys and girls,
So on my 74 valiant 4 door when she was started awhile back while in park the speedo pegged to 30mph while in park:banghead:
It did this for about a minute then the needle dropped to zero. Ever since it dropped to zero it does not work even when actually moving:wack:
Would this be a problem with the speedo itself or could it be something with the speedo drive gear in the tranny?
Any insight on this issue would be great and appreciated