Please read through this - Managing Transaction Logs[/url]

Once you get log backups running again you can do a once-off shrink of the log to get it back to a reasonable size. If there's no where on the local drives to write that first log backup you can write it to a network location or USB drive or, if recovering prior to current time is not required, switch to simple recovery and then back to full, take a full backup, shrink the log file then restart the log backups.