How can I download an app stuck on "loading"?

I was having problems with my instagram account, so i decided to delete the app so that I could download it again.

The app will not download, it says "Unable to download app".

I can delete the icon if I want, but even so when I try downloading again it says the same message, and it stays stuck in "loading".

I tried restarting my phone, connecting to WiFi, and signing out and back on to my iTunes account and nothing works.

Please help me out.