My store was open and I went out for a couple of hours, when I got back the store was closed. This has happened a couple of times. Before this started happening I upgraded to 4.4.2 and opened the store yet it keeps on closing for no reason.
You can't sell much if the store is closed.
Just to verify, you're seeing the store closed page and not the service unavailable page?
Have you cleared your cache just to ensure there's no dust left laying around?
I just went into the back end General Settings and the Close Storefront box was checked. It must have happened during the upgrade. I've been opening the store using access key and never noticed it. My bad..
Have you checked your site for malware?
The store closed property is in the DB and is only selectable via the UI in admin. I'm not aware of anywhere in the code where the store is closed other than when doing an upgrade. But nothing outside the upgrade_center controller.