You might just need an update. That usually works for me. I’d also suggest going to the forum at their website – often when lots of people are having issues, there will be lots of recent posts about it.

A firewall update may have caused it?

Which client are you using?

Are you familiar with torrents? I really wish people would give more indication about what they know already in these kind of questions. You’d never get any answers on a question like this on a more technical forum. Since you didn’t clarify, I’m going to assume you are new to torrents:

There could be a lot of reasons a torrent could stall, but the most likely one is the availability of the content. Do you know what seeds and peers are? In case you don’t:
> a Seed is a user that has a 100% completion.
> a Peer (or leecher) is a user that is still downloading, so anywhere between 0–99% completion.

You can see what I’m getting at here, you’ll need at least 1 Seed connected to you to get the last bits of your torrent. If you want to find out if you are connectable, check out the portforward site. You can also find guides there which help you figure out how to open the right ports (do keep in mind connectivity issues are a two way street, sometimes you can’t connect to a Seed because they don’t have the right settings themselves).