You certainly need to investigate why a database of that size has a log of that size. My point was only that if it won't truncate automatically, it won't truncate manually, either.

Use DBCC SQLPERF(LOGSPACE) to find out how much free space you have in your log file. This will give you an idea of whether you have a problem at this very moment. It may be that you have a long-running large transaction.

Please also read this article by Gail Shaw, which explains things better than I can:

http://www.sqlservercentral.com/articles/Transaction+Logs/72488/

John