How to enable multi-store in ftp or database

[Vins0n254]

Please help me may add to cart is not functioning, I think it happened when I turned off the multi-store in general tab settings.
so Im going to enable it again, then suddenly I can't access the general tab. it shows a blank page. then have already clear the cache but still nothing happens.

I also try to enable the debug mode but it gives an error when accessing general tab
"

ClassNotFoundException in AdminPreferencesController.php line 162: Attempted to load class "Order" from the global namespace.
Did you forget a "use" statement?"

i did not touch anything aside from css

im frustrated using prestashop 
Thank you so much