By default when installing SQL Server the built in administrators group is added into the sysadmin role, so as long as you/they can connect to the SQL Server instance with an account that's a local administrator on that machine, you should be able to troubleshoot appropriately i.e. (reset the sa password, etc)

As Superdoc mentioned, it would help if you could post the actual error (you can find this in the log file/sql error log file that's by default located in the directory...which is usually something like \MSSQL10.MSSQLSERVER\MSSQL\Log\)