I have an iPhone 4S running iOS 7. I noticed that neither the notification center nor the control center were showing up when I was swiping from top and bottom resp. At first, I thought I wasn't swiping properly. Then I was sure something was wrong. I restarted the phone by holding the sleep and home buttons. After the restart, everything was fine. This has happened to me twice. Unfortunately, I don't know what events cause this, since the effect is observed at random times. The cause(s) cannot be detected easily. Anyone else with this issue?