cannot sign in and make purchases

Yesterday, I was trying to make purchases via app store, when I clicked the "install" button, it appears the downloading circle for a while then goes back to the "GET" button. I signed out my Apple ID and I was not able not sign in again. I also tried the Reset, Reboot, Restart but still doesn't work. Can anybody help me to fix this? Thank you. I am using IOS 8.3