Do hamstrings ever heal completely after strained? as will i be able to be as strong and fast as before?

A muscle strain means some muscle fibers were torn but the muscle is still intact and connected to bones at both ends. Over time, with rest, the torn fibers will heal, then with gradual return to full exercise the tear can be strengthened and built back up. However if not allowed to heal first, the small tear can get larger and may develop scar tissue which can’t be as easily strengthened.