You get this error when opening your client: "The server is unavailable. Check your configuration and network connection and try again.". There are different things you can check in order to fix this issue:
1. The AOS service is running (check this in the Windows' Services console). If it's not showing Started you have to start it and retry.
2.a. Check the configuration you are using to connect to the AOS. On the client's machine open the MS Dynamics AX 2012 Configuration utility in Administrative Tools and make sure that the values are correct under the Connection tab, if you are opening the client directly using ax32.exe.
2.b. If you are using an .axc config file to connect to the AOS, make sure that the values in the file are right.
3. Make sure you don't have any issues with the connection between the client and server machine; check the firewall properties on both machines.
4. Verify that the versions of the AOS and the client match.