What is the recovery model of your database? Is suspect it to be FULL.

If your recovery model is FULL, follow the below steps

1. Take a full database backup

2. Shrink the log file to an appropriate size

3. Take a full database backup again

4. Schedule transaction log backups to avoid such issues in the future

You can also check the below mentioned article for more information on Managing Transaction Logs

http://www.sqlservercentral.com/articles/Administration/64582/