I think I've heard of cases where MPM either did not have the URL of a valid package mirror or the mirror in question was down, but I'm not positive.
Did you check to make sure that there was not a dialog box open and awaiting input behind some window? Will LyX start (so that you can run Tools > Reconfigure)? If not, try opening a command prompt in the LyX program directory and running 'python configure.py'. (You may need to supply the path to python.exe.) Does that hang up? Paul
