Check the answers on this post:
Why am I getting "Cannot Connect to Server - A network-related or instance-specific error"?
Then, if your SQL Server service needs to be re-started and will not re-start under the account you have configured, you may be able to select a different account provided you have Administrator rights.
To change the account under which the service runs, try:
All Programs | Microsoft SQL Server 2012 | Configuration Tools | SQL Server Configuration Manager | SQL Server Services , Right Click SQL Server (instance) , Properties, this account (radio), Browse, Advanced, Find … then pick the account you like … “NT AUTHORITY\LOCAL SERVICE
“, CLICK “Apply” << ( IMPORTANT !!! ) then try to re-start under the new account you just selected.
For an answer on which account to choose check this post:
https://serverfault.com/questions/217654/difference-between-nt-authority-network-service-and-nt-authority-system
Good Luck