What could be the cause of a good size crack in the block exterior wall of my house? I did find huge roots from my crept myrtle near this wall.

Cracks mean movement in the wall from settling or, in your case, roots lifting under the foundation. Dig down a few feet out from the foundation and cut the roots. This will stop the crack from growing but on itsown it may take years for it to settle back down at all. Patcgh the crack with mortar or cement caulk in a tube.