I have seen very similar behaviour with my work machine running Vista 32bit. Upon returning after a remote session the primary screen is off as if it is receiving no signal which makes it appear that nothing is happening at all. The secondary screen is on though and some input being taken, as evidenced by the mouse cursor being present and movable, but no keyboard input seems to be taken (I've tried logging in "blind" to no effect, and using the keyboard's sleep key).
I have found that sleeping the machine, either using RDC from another local machine or psshutdown
from another local machine, returns the machine to a more normal state when it wakes back up. If one of the front-panel buttons on your machine is set to make it sleep this may work for you too, though with my machine it does not seem to.
I have taken to sleeping the machine as I finish an RDC session instead of logging off which means the machine is ready to wake when I get back into the office (or I can wake it up again remotely using WoL from a machines local to it that I have SSH access to remotely).
I have no particular idea why this happens, but sleeping the machine seems to reliably work around the problem.