Very large Transaction Log File - When is it normal?
Log truncation frees space in the log file for reuse by the transaction log. You must regularly truncate your transaction log to keep it from filling the allotted space. Several factors can delay log truncation. One of the most probable reason - you didn't make log backups. You need to backup your transaction log and after that you will be able to shrink it. If you don't need log backups you can switch database recovery model to simple and shrink the log. 
Other possible reasons are a bit more complicated (for example, you may have some issue with database mirroring or long running transactions). You can find more details here:
https://docs.microsoft.com/en-us/sql/relational-databases/logs/the-transaction-log-sql-server?view=sql-server-ver15#FactorsThatDelayTruncation