Maybe you can tie a long string or rope to the corner of the tarp or plastic, then throw it over the greenhouse with something on the end for a weight. You could then just pull the cover over the top? Be careful not to break any more panels on the roof, though. Use something big enough to throw, but soft enough not to break the panels if you miss! A plastic flower pot with a little dirt in it comes to mind.
Better to try than to just let your plants die! If this works I'd put the plastic over the greenhouse, then put the tarp over top of the plastic for extra insulation, too. You could fasten the plastic down permanently to fix your roof problem once the weather warms up, and take the tarp off. The tarp will have grommets you can tie the rope onto, as well. Tie it down to something heavy so it won't blow off.
