The Short Version: Every now and then File Sharing on the target machine running OS X Lion quits working. I must disable and then re-enable File Sharing to get it working again. So far I haven’t found a permanent fix.
Today I discovered that I could no longer access shared files on my iMac from a MacBook Pro via SMB File Sharing. Both systems are running Lion. The file sharing was working only a day prior. This problem was rather odd. I could access file shares on other machines from that Mac without a problem. I could also access that Mac via Screen Sharing using the same credentials that File Sharing wasn’t accepting.
I’m not sure what caused this problem. The only major change I made on the iMac since yesterday was to run a Full Defrag using iDefrag 2. I don’t know if this actually caused the problem (or if it even could). While troubleshooting the issue I repaired permissions on the drive, but that didn’t make a difference.
Though the short name, originally established under Snow Leopard, for the account had been working I tried using only the long name. That didn’t work either. Every time I entered my credentials the login prompt shook to indicate it didn’t work.
Finally, I just went into the File Sharing Options, unchecked SMB (AFP wasn’t checked to start with) and saved this change. This turned off all file sharing. Next, I went back into the settings and re-enabled SMB File Sharing.
It worked.
Despite the fact that the system settings weren’t any different I can once again access shared files over SMB. My best guess is that something hadn’t been fully re-enabled. Perhaps during the defrag process some permission was changed that prevented the sharing from working. By disabling and then re-enabling the service it may have re-established the appropriate file permissions. Of course, that’s just speculation.
Updated 11/24/2011: The fix wasn’t permanent. The same problem cropped up again and, as before, the same solution also corrected the problem.
Updated 06/09/2012: Yep. It still happens despite various system updates that have been released since. Every now and then I’ll attempt to connect and then find that I have to disable and re-enable the service to make it work again.
Press Any Key 
I’ve had the same problem with SMB shares between my Mac and Windows machine since moving to Lion. It seems once a month I have to turn off sharing, then turn it back on again. Have assumed there must be a timeout built into Lion for this, as it switched off in the middle of a file transfer last month. Most curious.
I still have the same problem as well. It worked fine earlier this week but this evening when I needed to transfer just one file I had to disable and then re-enable File Sharing to get it working again.
With Lion, Apple has finally caught up to Microsoft in the department of releasing software that isn’t even compatibile with itself! I have been running into this problem as well – joy. . .
I’m so glad others are having the same problem… I was beginning to think I was crazy or something… every week I restart SMB file sharing to get it working again…
Thanks SO much for sharing this, I can’t seem to find any fix for this other than “restart” file sharing by unchecking it, then checking it.
Works everytime, but it’s a shame that we even have to do it… I’m running 10.7.5 BTW.
Cheers!