PHP MySQLI code not working

$q = "UPDATE users SET password = '6c55803d6f1d7a177a0db3eb4b343b0d50f9c111' WHERE id = '1';";
mysqli_query($dbc, $q);

Dot knows but some cause it keeps giving me the following error:

Warning: mysqli_query()

Expects parameter 1 to be mysqli, null given


Can anyone help me

Thanks in advance