Go to Connect Store and tap Connect again; you don't need to disconnect first or reinstall the app. Success shows green Connected. If it won't turn green, email [email protected] with your store name.
Two things this is not: (1) paying for a plan doesn't repair a disconnected store, so don't upgrade expecting a fix, though membership does control publishing (trial end unpublishes products; see Troubleshooting: my products aren't appearing in my store); (2) a blank white page or application error is a browser issue: clear cache and cookies (Ctrl+Shift+Delete, All time), try another browser, then email support if it persists.