1. I assume you're local admin group of server.

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)