Probably need more info.
Assume you can view the share by name.
Have you verified both the machine and the User are properly added to the Domain? - Open the ADUC and verify the machine exists, and you’ve logged in with a Domain User account. Also, is the User account a member of the SBS Users group like any other User who can write to the Share?
That is… unless you’re trying to do something like open up the share permissions to “Everyone.”
So, that’s what I would recommend at least to start troubleshooting… if nothing on the Server has changed then I would look at
Whether the client machine is a proper member of the Domain
Whetheer the User account is really a Domain User account which works on a different machine. And, I really do hope that you have an obviously different naming scheme for Local vs Domain User Accounts no matter what OS to avoid mistakes that might be hard to detect.
With the logged in User, can you access any other SBS resources, eg if ISA server is installed and restrictive can you access the Internet? Can you pick up or view mail if using an SBS mailserver (OWA or POP/IMAP)?
Answering these questions can narrow down whether your problem is really file write permissions or a more general authentication issue.