Hi Michael,
This is terrible news. Unfortunate you cannot use this T-Log file with any other database files.
In addition, the T-Log only contain transaction information between the last T-Log backup and the current point - assuming yet the T-Log file is not corrupted yet.
1) Can you bring the database to a ONLINE state at least? If yes, run a DBCC CHECKDB with no_infomsgs against the database. Check the results, depending you may be able to recover the database from the corrupted state.
2) If you can't bring the database ONLINE or the database corruption is so severe (like system tables are corrupted), then your best option would restore the database. For this there are several options to follow. Data loss or not, depends on how good is your backup strategy.
e.g.
Having a very old FULL DATABASE BACKUP, does not mean that you will face data loss. In case you have all T-Log backups stored as well, you can stat your recovery from a DATABASE BACKUP taken from 3 years ago (like I said, you just have a start point, and if you have all the log chain backups since then you can until the last time your database was in a good state, you can still recovery the database.
Yuksel gave you some of resources about backup/restore. I would go further and suggest you to read on the following Blogs:
Corruption Handling in SAP Databases | Running SAP Applications on the Microsoft Platform
and also Mark Weber's blog:
General SQL Database Repair for Enterprise Applications | Mark Weber's blog
You may also contact Microsoft support. They are best skilled for handling database corruption, they offer 24x7 support (follow-the-sun) and if you don't hold contract with their support, you can still use an option for "pay-per-incident". I can say that for a database corruption no SAP skills are required for handling it.
Since the root cause of this (as you explained) is a security issue, not much I can suggest you to avoid this in the future other than improve Database Server security as well isolate the access to it. Dedicated database servers have no need to have end users accessing their file systems or network shares.
For the future, consider an efficient backup strategy along with some HA/DR solutions that would have help you to avoid all other corruption possibilities.
Best regards,
Luis Darui