One of my crepe myrtles fell over in all the wind and rain from Tropical Storm Allison (we had standing water for three days and the ground was too soggy to upright the plant alone). The tree was about five years old and about ten feet tall. The roots were exposed and out of the ground. My husband stuck three metal stakes in the ground and ran soft stretchy material from the stakes to the crepe myrtle (panty hoses) and anchored it. We left the tree like this until the following summer. It recovered fine. Good luck -