1989 50 HP FORCE WILL NOT GO FORWARD

First off,thanks for letting me do this.Went fishing the other day, everything was going just fine until I got ready to leave, started the motor up, put it in gear and it would not go,the motor is a 1989 50 hp Force,all it would do is just rev. up,put it in reverse and it worked just fine,sure hope somebody out there can help me on this problem.