Because you've been a bad boy. Your ISP recieved a complaint from some intellectual property owner, or they simply noticed that you've been hogging a huge amount of bandwidth and are costing them a lot of money, so they blocked your bittorrent traffic - some ISPs do that. Bad, bad boy
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OK, the other possibilities: download and install another (free) client. I recommend trying Azureus, because it has an inbuilt free diagnostic utility (of sorts) configuration wizard and NAT/Firewall tester. It's a lot easier and faster to test various ports also.
If Azureus works, you know at least that it may be your client. Also you give too little info. What kind of connection do you have? Dialup? Cable? DSL? Sat? T3? ISDN? What about the addressing... Static? Dynamic? Are you behind a router? How are you connected? Wireless? How do you run your torrents (encrypted?)? Is this happening all over, or just on some specific site/torrent - your IP address may have been banned (maybe you're a bad boy?).
Don't discount hardware and other software failure - your router may have gone wonky, or your firewall went berserk (do you have one in your router in addition to the OS X one?). If you're behind a router, try hooking directly into your modem and run again on your old client and the new client. Try to isolate your problem, and you'll be closer to a solution, or at least you'll have more info to post here so someone may be able to help.
Good luck, I know it can be frustrating when you really need that download
