This happens fairly regularly at a client site with a mixture of XP and Vista PCs.    It’s common for users to remote from an XP machine to a Vista machine, only to find that when they return to the Vista machine, the screen is blank (black) and there’s no way to bring it back to life without switching it off and rebooting.

Believe it or not, it appears that the problem is related to smart card authentication (which isn’t even used on the site).   On the machine establishing the connection – go into the remote desktop properties, under the Local Resources tab, press the More button.   If you deselect “Smart Cards” – the problem is fixed

There is a long discussion of the problem with other possible solutions over at if the above fix doesn’t work for you.