What happens when you try to open an app? Are you getting any sort of error message?
Without knowing any details it's hard to say what the issue is, but a common problem with many Android phones is a lack of onboard storage. When you don't have much space, it's easy to fill it with just a few large apps and that can cause problems with apps running properly, especially apps that require additional storage to function like photo and messaging apps. Check to see if your phone's storage is full, and if so go through and delete some apps or files (especially photos).