If you're still having issues with the homepage while being logged in: Close the Steam client app. Go to the preferences section of the web browser version of Steam (https://store.steampowered.com). (Log in and click the name dropdown at the top right, then preferences). Try ticking and unticking some of the browsing/store filter/cookie options then going back to the homepage until it starts working.
I think they have some auth cookie/profile issue since the homepage loads just fine if you're not auth'd. Once it's working on the browser, it should be OK on the client too. Whether this shit is actually broken or you just need to get a successful login I'm not sure. Only the population of the main sale homepage was busted for me, every other page I could navigate to via the top menu was fine.