What do you mean is the actual obstacle for you contacting her? Is it beacuse you don´t want or dare to or is it because you don´t know how to get in contact with her? Why she left is one of the most important questions to get an answer to as I see it. As it seems she left quite abruptly, it could perhaps be about an illness in her family, a divorce, having to leave her home or something like that.

If she worked in an agency together with several other T:s you could perhaps ask one of her colleagues or a receptionist why?