Although most hoophouses can withstand some snow load, if you've got too much snow, you need to get it off. You can attack it from the inside first, but if that doesn't work, you'll have to go outside.
From the inside: Using a broom, sweep down the sides of the greenhouse poly to dislodge the snow. You've got to start at the bottom and work your way up as the snow cascades down. A wide push broom will remove more snow per sweep, but it gets heavy holding it above your head for any length of time. You may prefer a lightweight kitchen broom. This sweeping usually works just fine when the snow is light and dry. But if it's heavy and wet and won't slide down, you need to go outside with a partner.
Outside: Get a length of rope long enough to extend across the top of the hoophouse. Tie knots in it every couple of feet, and throw the end across the hoophouse to your partner. Then just pull the rope back and forth between you; the knots help dislodge the snow. This is when it gets really fun, as sheets of snow cascade down into your boots!