How to: Repair a SQL Server 2008 R2 Installation
Repair operation can be used in the following scenarios:
- Repair an instance of SQL Server that is corrupted after it was successfully installed. 
- Repair an instance of SQL Server if the upgrade operation is cancelled or fails after the instance name is mapped to the newly-upgraded instance. - If you see the following message in the summary log, you can repair the failed upgrade instance: - "SQL Server upgrade failed. To continue, investigate the reason for the failure, correct the problem, and then repair your installation." 
- If you see the following message in the summary log, you need to uninstall and reinstall SQL Server. You cannot repair the SQL Server instance. - "SQL Server upgrade failed. To continue, investigate the reason for the failure, correct the problem." 
 
When you repair an instance of SQL Server:
- All missing or corrupt files are replaced. 
- All missing or corrupt registry keys are replaced. 
- All missing or invalid configuration values are set to default values. 
Before you continue, for SQL Server failover clusters, review the following important information:
- Repair must be run on individual cluster nodes. 
- To repair a failover cluster node after a failed Prepare operation, use Remove node and then perform the Prepare step again. For more information, see How to: Add or Remove Nodes in a SQL Server Failover Cluster (Setup). 
To repair a failed installation of SQL Server from the Installation Center
- Launch the SQL Server Setup program (setup.exe) from SQL Server installation media. 
- After prerequisites and system verification, the Setup program will display the SQL Server Installation Center page. 
- Click Maintenance in the left-hand navigation area, and then click Repair to start the repair operation. 
- On the Language Selection page, you can specify the language for your instance of SQL Server if you are installing on a localized operating system and the installation media includes language packs for both English and the language corresponding to the operating system. For more information about cross-language support and installation considerations, see Local Language Versions in SQL Server. - To continue, click Next. 
- Setup support rule and file routines will run to ensure that your system has prerequisites installed and that the computer passes Setup validation rules. Click OK or Install to continue. 
- On the Select Instance page, select the instance to repair, and then click Next to continue. 
- The repair rules will run to validate the operation. To continue, click Next. 
- The Ready to Repair page indicates that the operation is ready to proceed. To continue, click Repair. 
- The Repair Progress page shows the status of the repair operation. The Complete page indicates that the operation is finished. 
To repair a failed installation of SQL Server using Command Prompt
- Run the following command at a command prompt: - Setup.exe /q /ACTION=Repair /INSTANCENAME=instancename