Office Network All Over the Place

This morning I came into the office to discover that e-mails were not working, I looked to see why, and noticed that all User Mailboxes have been disconnected, On further inspection I noticed the password to our server was changed... Essentially it looks like our network has been hacked. I went ahead and downloaded password cracking tools for our main machine, cracked the password and I am able to login into windows server.. However none of the files are there and the IP has been changed, strangely enough, there is still 1 user in the office that is still able to access the mapped drives, and I can ping the IP of the server just fine using the OLD ip.. but the machine does not have that IP anymore.. So the network still recognized the old IP and asks for credentials *That are not working* and i changed the machines login information, and cannot access the drives using those credentials. PLZ HELP