Is it for some particular app or all the apps?
1. You might have disbled notifications in the app itself. Check the app settings if notification setting is hidden somewhere and enable.
2. You might have disabled notifications from Android App settings. Goto settings>>apps(I do not remember exactly, but it should be settings, mostly settings>>apps). Select the app you want and make sure "notifications" box is checked. If it is not checked, Android does not display notifications for that app.
3. There are apps available which disable all notifications. Did you ever install such an app? check, open and enable notifications if such an app is installed.
Hope this works. Let me know if anything worked.