My guess is your database is in the full recovery model and you are not making log backups.

The short answer is that you should:

1. Backup the transaction log with truncate_only

2. Make a full backup immediately

3. shrink the log file with dbcc shrinkfile back to a reasonable level

4. Set up log backups every hour or two.

You ought to read about how to manage transaction logs as well: http://www.sqlservercentral.com/articles/64582/