Hello,
my problem was that I get blank page after manual upgrade from 2.8.0/2.8.35 to 3.x. on localhost (Windows).
Also there is no errors inside log files (php, apache... site/logs).
Problem solved after addition in top of config file (site/config.php):
namespace ProcessWire;
One of the first thing what I done (after upgrade and blank page) was that I added this in site/config.php :
// no need to add this!!!
// disable module compilation
$config->moduleCompile = false;
// disable template file compilation
$config->templateCompile = false;
but did not pay attention on config file, and "configure" itself .
Regards.
p.s. maybe this should be noted in this page