The pawls in your freehub aren't engaging. First, try warming up the bike. If that doesn't work you'll need a new freehub. If you have the right tools (and a new freehub) you can change it yourself, but your Local Bike Shop may be the better part of valor.
21 speed Altus level bike is almost certainly a freewheel, not a freehub.
To the OP,some light oil dribbled in the gap between the part that rotates, and the part that's stationary when you spin the gears backward will likely solve the problem.
Worst case scenario, new freewheel, 15 bucks.