Check to make sure that the network is set to "Home" or "Work". If it is set to "Public", then the file sharing services are shut down right down.
While I understand that another computer is currently connected and able to use a share from your problem computer, I reckon that the network site on the computer has changed.
If you go in to the Network and Sharing Center, you will find most of the sharing settings on the left and you will also be able to verify the connected network type.
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