SafeGuard can't log in

Are any of their security officers bound to a domain or is this the ONLY user to log in with that’s known to the organisation? 

thank you.

I confirm with the customer whether the domain is bound. If there is no bound domain, what should I do next?

I would open a support call with Sophos first off, best having them on your side first! You'll need to have a copy of your certs AND the passwords set to over-ride the cert store password. They will need to have a good search on their server and see what WAS backed up/written down intially. This article may help a little - Sophos 5.6 console - forgot password and MSO, any way to recover? - Discussions - Encryption - Sophos Community

I would open a support call with Sophos first off, best having them on your side first! You'll need to have a copy of your certs AND the passwords set to over-ride the cert store password. They will need to have a good search on their server and see what WAS backed up/written down intially. This article may help a little - Sophos 5.6 console - forgot password and MSO, any way to recover? - Discussions - Encryption - Sophos Community

Ok, thank you for your reply, I refer to the link you sent to check first, I hope it can solve my problem