How to Trouble Shoot My Network

I hope someone can elp me I am far from a network admin. Today I come to work and as the assigned geek get told to solve our network problem. My network consist of a application/file/web/mail server with 1tb HDD divided accordingly. We have 5 work stations(clients) that access and file share each others work. I have a blackberry that can recieve MS Exchange e-mail. Some of the network is both wired and wireless. When the first person came in today the server and all clients were all down and not able to obtain internet access or communicate with the server. They unplugged system and reboot but were unable to restore any client. The main server can now access the web and seems to be recieving e-mail but is not sending or recognizing clients nor is the network able to send out any e-mail. I am not a CNA and don't know where to begn to fix the problem. We recieve are ISP from another networks modem and there system is working with no problems reported. We are just on LAN on their WAN. i hope someone can tell me what I should do to restore our small network? Thanks for your time INEXPERIENCE Bill