For example, all pages in the site, files in document libraries, files attached to lists, and information in lists are stored in the content database, and security and permission settings along with other configuration settings are stored in the configuration database in SQL Server. SharePoint Foundation 2010 uses a service account to communicate with the database on behalf of a user request. This service account can be either a specific user name or password (domain name and password), or a predefined system account, such as Local System or Network Service. When a SQL Server database is created, a value for the maximum database size is set. Each database has a separate database size setting. Note that a Web application might be associated with one or many databases.
This error indicates that Microsoft SharePoint Foundation could not connect to the SQL Server database.
One or more of the following symptoms might appear:
• Attempts to communicate with SQL Server fail and no content from databases hosted on the SQL Server can be accessed.
• This event appears in the event log: Event ID: 3355 Cannot connect to SQL Server.
One or more of the following might be the cause:
1. The SQL Server might be offline.
2. The SharePoint Foundation database access account might not have the necessary permissions to communicate with the SQL Server.
3. A firewall that runs on either the local server or on SQL Server might be blocking network communications.
Troubleshooting Steps:
Grant correct permissions to the database access account:
You must be a member of the Farm Administrators group to perform this task.
Assign the database access account:
a. On the Central Administration home page, click Security and in the General Security section click Configure service accounts.
b. On the Service Accounts page, in the Credential Management section, in the upper dropdown list click the correct Web application pool for your Web application.
c. In the Select an account for this component drop-down list, click the domain account that you want to associate with this Web application pool, or click Register new managed account to associate a new domain account with this application pool.
d. Click OK to save changes.
Verify that the account has correct permissions in SQL Server:
a. Connect to the computer that runs SQL Server by using an account that has Administrator permissions.
b. In SQL Server Management Studio, Object Explorer navigation pane, expand the Security node, and then expand the Logins node. The name of the database access account indicates that it is a SQL login.
c. If the account exists, in the Object Explorer navigation pane, expand the Databases node, expand the configuration database node, expand the Security, and then click Roles.
d. Expand the Database Roles node, right-click db_owner role and select Properties.
e. In the Database Role Properties dialog box, check whether the database access account is in the Members of this role list. If the account is not listed, click Add.
If you have any queries or questions regarding the above mentioned info then please let me know, Thanks...
