There are numerous possible causes for this issue, you need to start eliminating specific items. Here are some things to check.
Check that all services are running.
Check event logs for application and system on the server and make sure no messages.
Verify that TCP/IP Protocol is enabled on the server.
Check windows firewall.
Can you connect to the sql port from a client machine? Try to ping the server. If it responds try to telnet to the IP address on 1433 (if you are using the default port) and see if it connects.
Can the server see any of the clients or other machines?
Let me knwo the results and I cna maybe help further.