Many of my apps will not open

Today, When I touch the screen on my iPad 2 to open a new app,, the app. did not open.I tried tapping the

screen many times to make it open but it doesn't. I will then try to open many other app & they will not open

either. This did not happen to all the apps but many of them will not open. All of these apps have all opened

in the past, but today they will not. This has happened many times in the past. I've tried a complete shut

down (holding down the button on top of the iPad) 3x & it still didn't help. Any suggestion on how to solve this problem.