Login failed for user 'DomainName\SQLAdmin'. (.Net SqlClient Data Provider)
Go to Start, Programs, SQL Management Studio -- Right Click on that and say Run as Administrator. Now pass your credentials.
This will allow to you open SSMS for your login(SQLAdmin).
But if you need to fix permanently The user SQLAdmin must be part of Local administrator group of the server and should have full rights to access SQL server installation path like C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER