Attempting to upgrade from 7.5.4.2 to ver. 7.5.6

I followed upgrade instructions as always and received the following error msgs.
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SQL Error: Unable to connect to database server, please try again in a few minutes.

SQL Error: Unable to connect to database server, please try again in a few minutes.

Fatal error: Call to undefined function upgrade_header() ......... upgrade.php on line 67

thanks for help.

Charles