There is a comprehensive backup solution in place, so you should not worry about your data being lost forever. For Salesforce Marketing Cloud, customer data is automatically replicated on a near real-time basis at the database layer and is backed up on a regularly to a SAN array. Database transaction logs are backed up every 15 minutes and at least one complete backup set is kept on disk at all times.
Backups are encrypted using strong industry standard encryption as they are stored to tapes. Backups stored on backup media are encrypted and retained for up to 90 days, after which time the data is overwritten or deleted. Backups are verified for integrity and stored at Salesforce’s data centers and/or secure off-site location.
Customer databases are backed up, but individual messages (email, SMS, push notifications, etc) sent via the platform are not.
Salesforce Marketing Cloud performs backup and restore activities to validate associated processes on a scheduled basis. Testing or exercise of backup activity is performed on a quarterly basis.
In case of data being corrupted on your DB, Salesforce will probably be the first to know, and notify you while steps are being taken to restore it from one of the backups available.