Network Sharing magically breaks

Goof

Hi all,

I've got a small network, with one central Win7 machine acting as a file server and a few client machines. I set up the network about a year ago and everything's been working fine since.

Last week, everything worked as normal, shutdown for the weekend, turned it on Monday morning and none of the client machines can access the network

I've tried everything I can think of/find online but nothing seems to be sorting it (sharing, advanced sharing, security, permissions). The server shows in each client's network (and I can ping it from each) but when trying to access via Windows they all give the "Windows cannot access \\server" message.

The exception is one other Win7 machine that can still access it fine. I thought it was because, in an attempt to fix things, I had added both of them to a homegroup. However, today I tried 'leaving' the homegroup on both machines, weird thing is that client can still access the server.
So I'm guessing still a permissions issue?