I suspect the "log file" is a file which is part of the SQL database - I think that it is a "transaction log" file (as opposed to any kind of diagnostic log). It sounds as if you have not been correctly backing up and maintaining the SQL database which is associated to lthat log file.
The transaction log file contains all changes to the SQL database, is used to ensure integrity of the database.Normally when the database is correctly backed up, it is cleared down . If SQL databases are not backed up correctly, that file wlil grow and grow...As part of a full SQL backup, (the file size of the log file does not shrink, but the data inside is deleted, and after the backup, the free space gets reused.
I'm not a SQL server expert (so I can't talk you through ste- by-step), but basically you need to do a full backup of the SQL server databases (through an SQL aware backup program, or through SQL server management studio). As part of the backup, you need to specify that the log files are truncated. Then once done, you can use SQL server management studio to "shrink" the now empty log file. Note that a normal file/folder backup wil not backup SQL databases, or manage the transacion log files.
If you have support on the program that uses the database file, the support company might be able to help you do the backup, truncation, and shrink, and then set up a maintenance plan to backup the databases abd maintain them periodically,