1 -stay calm.
2 - what is your database's recovery model? If it's simple, see 3). For full and bulk-logged, see 4).
3 - two possibilities
3a. you have an old transaction that's still active, that's preventing the log from being truncated, causing indefinite growth. Use DBCC OPENTRAN to determine if this is the case. Take steps to end that transaction (commit, rollback, kill etc). Then use DBCC SHRINKFILE to shrink the log file. May need to do it a couple of times, see here
3b. there were large transactions that caused the transaction log to grow to such a huge size. If this is going to be common, shrinking the file may not be a good option. If not, run DBCC SHRINKFILE to shrink the log.
4. see 3a) and 3b) for possible reasons (but not the suggested resolutions), also
4a. you have not been backing up your transaction log, hence causing the transaction log to grow indefinitely.
Do you need to make a backup of the current transaction log, perhaps for log shipping purposes, or to be able to perform a point-in-time recovery (note that this would still be impossible if there are minimally logged transactions in the current transaction log)? If yes, you'll need to back it up to disk. Run a BACKUP LOG ... command, folowed by DBCC SHRINKFILE.
If not (and you're very sure of this), run a BACKUP LOG with the TRUNCATE_ONLY option, then follow the suggestions by Hadrian and Andras, especially the part on performing a full database backup.
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