I've had this problem once or twice. The best way to fix it is fix it in the CAD software if you've built it.
The lesser solution if it is not your model to monkey around with, use the rotate feature under double clicking the object. First look completely side on to the object from a plane view (either rotating the screen or using the view options). Then double click on the model and start trying random degree rotations. I often have to do this when I model from the wrong plane in SolidWorks and need to rotate it 90 degrees. I have been able to fix 1 or two degree rotations.
Further, I have had some models that inexplicably "float." My solution has been to use the position coordinates and slowly lower it down until it touches. Do note that if a part of your model is "below" the bed plane, it won't be printed.