do you mean the log files that the server itself writes? Those have no file extension normally, usuall called something like ERRORLOG. when the server restarts it renames it to ERRORLOG.1 and starts a new one. I would be very surprised if you have a 20gb one though. that's pretty large. maybe you have a lot of failed login attempts (which are logged as I recall).
Or perhaps you mean a ldf file? These are transaction log files for a database - it's common for these to get large. You can shrink the size of the ldf using DBCC SHRINKFILE. You should not delete an ldf file unless you want to delete the database it's associated with.
Note that if you care about recoverability, you should be making regular backups of both your database and transaction log.