If I undertsand what you say:
This is just Windows Live Messenger's default setup. There should be an option to "Group Sequential Message from each contact" or "Show timestamps before every message" or something similar. It needs to be checked to go back to the old behaviour
If you mean it doesn't send the message either, but is still in the text area... then you might want to check that the shift key isn't stuck
W00t - 1000th Post