When you say you've connected remotely before without problems, I assume you're talking about some other connection besides TeamViewer.
TeamViewer has to be running on both machines. On the machine you're sitting at, look at the "Computers & Contacts" pane. That shows the list of computers that your system knows about. Is the remote computer that you're trying to reach listed there and identified in the "My computers" column? Those are the ones it recognizes as being "online". Or is it in the "Offline" column?
If the former, and the ID for that computer is listed under "Partner ID" in the remote control pane, it ought to connect. Are both of these machines local? Any possibility that the one is hiding behind a router gateway that won't allow the connection?
If the latter, then you'd better figure out why. Probably because