The report server cannot open a connection to the report server database. A connection to the database is required for all requests and processing.

start cmd

Solution:
rsconfig -c -s < SQLSERVERNAME > -d reportserver -a Windows -u < MYDOMAIN\MYACCOUNT > -p < PASSWORD >
or
rsconfig -c -m < REMOTECOMPUTERNAME > -s < SQLSERVERNAME > -d reportserver -a SQL -u SA -p < SAPASSWORD >

rsconfig Utility

The rsconfig utility (Rsconfig.exe) encrypts and stores connection and account values in the RSReportServer.config file. Encrypted values include report server database connection information and account values used for unattended report processing.

Configuring a Report Server Connection

After installation, you may need to modify the user name, password, and authentication mode that the report server uses to connect to the report server database. These values are encrypted. Reporting Services includes rsconfig utility that you can use to modify encrypted connection information.
Connecting to a Report Server Database

A report server must access the report server database that stores the items and metadata managed by the server. Because the report server database is a SQL Server database, you can use either Windows Authentication or SQL Server Authentication.

The connection information used to establish the connection is initially defined during setup. However, if you want to modify the connection information, or if you are moving components to different computers, you can run rsconfig to correct it.
Running rsconfig

The rsconfig utility is a console application that you run from a command line. You can run this tool on a local or remote computer to modify the connection information used by a report server instance.

When you run the utility, you specify parameters to set connection information. These parameters change the values of the connection information stored in the configuration file on the local or remote computer. You can modify the user name, password, and authentication mode used by the report server to connect to the report server database. The tool manages connection information for a single instance of the report server.

links:
http://msdn2.microsoft.com/en-us/library/ms162837.aspx

http://msdn2.microsoft.com/en-gb/library/aa972232(SQL.80).aspx

http://sqlforums.windowsitpro.com/web/forum/messageview.aspx?catid=92&threadid=29121&STARTPAGE=1