I'm running a linux (centos) server wiht samba in a windows domain environment. The server is joined to the domain, and certain users have access to various directories on he linux server, all determined by the config file.

Users also were able to access their linux home directory (/home/username) from windows by going to \\server\username. Each user has a linux username corresponding to their domain username. For some reason, users are now unable to access their home directory. All other shares are accessible by the appropriate users.

I looked at some logs, and it seems that when a user tries to connect, they are attempting to go /home/domainname/username. However, the directory /home/domainname does not exist. I tried creating it - still can't access.

No changes have been made to the config. Any reason how/why this would happen?

That's how windows domain users log in.
Domain/User->password.
The windows login plate obscures it, but try to add an unknown domain user to a local machine and you'll see what I mean, it prompts for the domain.
Probably the easiest fix is to move the home directories to where windows wants them. Any changes you make to the credentials the Windows guys use for domain access is liable to have unpleasant side effects. You could create a second set of credential on the Windows machines to access the Samba box but Windows is confused about security on it's best days. A third option is to have them map by UID instead of login username, which is a really nice functionality but it can be tricky to set up and implement on a short ramp.

Seems to work, thanks!