Why won't my 1980 70hp Evinrude Shift into Forward or Reverse ?

-Today was the first time I took my boat out since fishing opener. I did not tune up anything over the winter and went for a run today on the lake. Everything started out fine, but than 15 minutes in my reverse went out. When I shifted into reverse it would just start revving but the prop would not spin, so I only could go forward. After another 30 minutes my boat would not go into forward, when I shifted into forward it would do the same thing and rev but the propeller would not spin. Kind of sucked because I was on the middle of the lake and had to row into shore and ask a guy to tow me back to the launch :(

Any ideas what this could be ?Just a thought. I had a motor like yours years ago. It was selelctric shift. Perhaps the battery is low and the shift will not work. Worth a try.