First step is to check the basic IP settings. Double-check in particular the IP address and subnet mask of the server and ensure that it's in the same subnet as the clients. If it's not (and if it shouldn't be), make sure that it's set up correctly for whatever subnet it is in (a correct IP address for the network, a correct subnet mask, and the default gateway set to whatever the router for that subnet is). It's probably worthwhile ensuring that the NIC isn't disabled in Device Manager at this stage, as well as ensuring that you have the correct NIC (if it has more than 1) connected (been there, done that).
Then, as others have mentioned, try a
ping. If that doesn't work, try using
tracert to see where connectivity is failing. Address any issues that this throws up. Also try switching off the Windows firewall (at least to rule out possible complications arising from it being on). Move it between switch ports, and if possible put it on the same switch (and same VLAN if you're using VLANs) as a client you're testing from.
Next step is to try connecting to the share. All going well it should work, but if it doesn't you can use
\\serverip\c$ if you don't have it in your DNS) as an administrator to ensure that the basic Windows File & Print Sharing service is up. If that doesn't work you need to check that File & Print Sharing is bound to your adapter and that the Server service is started. Repeat until it works.
Now try connecting to the share again. Assuming that everything else is working, the most likely remaining problem is that your share permissions don't allow access to it.