It sounds like you did the best you could have done under the circumstances. However disconnection problems like the one you describe are unfortunately all too common in peer-to-peer arrangements like that. The key thing you need to explain to them is that with a peer-to-peer network there is no centrally enforced administartion or control - all users have equal rights so any of them could have gone on to that PC and inadvertently undone your work (although they'll all deny it). That's not a flaw in your work; it's jus the way peer-to-peer arrangements are. If they want to be sure it will work every time they need to go for the greater predictability and control of a domain-based arrangement. You could have tried to sell them that; but it probably wouldn't have been cost-effective for such a small company so by working with what they already had you did the right thing. To solve the present problem you could put them a batch file on each PC run by a simple icon that reconnects the shared drive and the printer, and tell them "If ever it doesn't work, double click this". Alternatively you could just document the procedure for re-establishing it all and give them that. You probably should do something - if you dump them they will write you off, but if you respond professionally there might be more work for you in the future. And finally, if they switch off the entire network then of course nothing will work, but there's no reason why it shouldn't work again when they turn it all back on. And, of course, if they turn off (or don't bother to turn on) the machine that is doing the sharing then none of the users will get the shared drive or printer. Again, the same comments apply here regarding this being an inherent feature of the peer-to-peer arrangement.
Hope that helps and is what you were looking for!
Just to add to what bishop has said already. I would recommend that no users actually use the PC that file/printer shares. It's common in small business that people use the "server" for everyday use. People get e-mail, d/l software, change settings, and surf the web. Like bishop said the users will break stuff and deny it.
Some more things to try.
1. Make sure the "server" has a Static ip address outside the DHCP range.
2. Network drivers and Printer. Since its peer to peer and its unreliable I usually use Ip's to make network drives\map printers.
\\<server name>\<share name>
3. I've already mentioned this of course but keep users off the "server" this always helps TONS.