How to compensate for yardage drop with a cross bow?

I own a horton Explorer 150 lb cross bow, It seems to shoot dead on from 10 yards up to about 30. When I tried to shoot at 40 yards it completely missed the target and hit the ground in front. Any ideas on how to fix this problem? I really don't want to adjust my sights because season opens in two days and I don't want to mess everything up.