Try these steps from EvilDBA
1. Lets assume you're local admin of server or domain admin.
2. RDP to the server.
3. On command prompt NET STOP MSSQLSERVER (stop sql server)
4. On command prompt NET START MSSQLSERVER /m (start server in single user mode)
5. Now login to SQL server.
6. Created a new login on SQL server and make it as SYSADMIN.
7. Exit from sql server.
8. NET STOP MSSQLSERVER (stop sql server)
9. NET START MSSQLSERVER (start sql server)
Thank You,
Best Regards,
SQLBuddy.