Mostly new user to magento commerce encounter problem on migration or switching to new database.

Here is some tips how to avoid that:
1. Always backup your database.
2. Make sure you know what your doing. Consult to Magento forums if needed.
3. Go to app/etc/local.xml, look for this code

<connection>
    <host><![CDATA[localhost]]></host>
    <username><![CDATA[root]]></username>
    <password><![CDATA[password_here]]></password>
    <dbname><![CDATA[your_database_here]]></dbname>
    <active>1</active>
</connection>

– You can now update your own database settings.
4. Final thing to do is clear or delete magento’s cache files from the directory: var/cache.