When I have had a second monitor attached to my laptop, sometimes PT screws up the starting position. This is because it tries to open in the area of the combined screen where the other monitor was.
The only way to fix this is to try some moving around with the monitor attached and hope to find a place where it isn't off-screen next time I start it, or to run PT maximized by right clicking the task bar entry, or to edit the registry settings.
It's a common bug among software that tries to open in the same position every time. And usually it's silly, because almost no-one will want it in the same position next time they open it. Remove the code for it and let Windows handle the placement.
JFYI, skype has the same bug
Bo Thorsen.