I've just got a new iphone and had no idea that sometimes they go a little loopy when updated with IOS 11. It can cause the App store to not be able to open AT ALL. Extremely frustrating when you're trying to download a new secure app, which doesn't get automatically transferred from your old phone because of its security features.

Anyway after a ridiculous amount of research when I should have been being creative, I finally managed to fix the problem. Annoyingly, I first tried EVERYTHING else that was suggested on fix-it websites, and then when nothing helped, I just took a chance on a whim.

So, just in case you have an app store refusing to open after an update, here's what I did, all of which didn't work for me, except one, but they've worked for others.

Turn it off and on again - Sounds simple, but sometimes the simplest solutions work best. Turn your phone off and on again and it might work for you.
Sign out of your itunes and app store - This might work, go to settings, look for itunes and app store - which is in between privacy and apple pay for me, and sign out. Then click the App Store icon and if it opens, keep it open and log back in via settings.
Check your date and Time Settings - This is a weird one, but if you've just updated your new late-night online shopping purchase, and the App Store isn't working, it's probably because it's not synced with the rest of Apple. Try to get your settings manually set and retry.
Reset your Network Settings -  Go to Settings, then General, Reset and reset network settings.
Check your wifi settings - Make sure you're connected to the correct server and have all the correct passwords
Turn off your wifi and use mobile data- Okay this is the one that worked for me. Makes zero sense, but it worked and actually once you've opened the app store, you can actually connect back to wifi and the app store will continue to work as normal
Reset your entire phone settings -  As a last resort obviously and as long it's all backed up, this should be fine. I actually tried this because my phone was new, so it wasn't too horrific a transition, but it didn't work. It was only when I used my mobile data instead of wifi that it worked for me.
Good luck, and if you have any solutions for this problems, do give us a shout. Also, if you're looking for a new iphone, check out these guys... (link to sellers - later you can get them to use this advertising space)