I have been dealing with this same problem on my Live Plus the past few days, basically for the first itme ever in well over a year of working perfectly. My main server PC that setup my network is a Win7 64-bit. I found a solution it appears for the past few days now: REBOOT the Win7 PC and problem was solved.
Now, here are the details of the solution. I noticed shares seemed to not be there a few days ago except for the Win7 PC. I knew sharing was set right, as the Win7 and all other network shares of all other PCs and printers, along with the Win7 were working between themselves; the problem was at the Live Plus TV display. After checking stuff, I decided to put the newly-purchased used Live Plus I bought for a spare in place of the original one. Problems remained the same. So, not the WDTV at fault.
After more head-scratching, I figured the problem HAD to be coming from the Win7 PC, and somehow it was blocking all shares at the Live Plus. So, with the original Plus back in the loop and while it was still on, I rebooted the PC, and once the reboot was good and over with, I looked at the TV, and all network shares were back again.
I attribute the cause to possibly originate from a recent Win7 update (I have experienced computer problems in the past after updates, and a month or so later they are gone (likely due to a fix in a susequent Win update).
Also, other people here have recently described having a similar shares problem that just began; again making me suspect a Win7 update issue.
Nonetheless, I recently added a Pogoplug with software and hardware to the network, and it has its own particular issues, so I will check to see if it could be the cause of all this, but not likely; it appears to the Live Plus as a new server (not a share).
Or, it could just be due to sunspot activity. Heck, who knows! :-)
When all else fails, don’t start tampering with settings that worked; and should still work; just REBOOT the PC (or WDTV, etc).