Nothing has changed. Have you looked at your Play Store settings and maybe changed something accidentally? Or made changes to your Google Play services app on your phone? If you disabled anything to do with Google on you device it may prevent you from getting notifications. Also check your sync settings under Accounts & Sync. Make sure you are still set to auto sync and that Google is still listed there. With the Play Store open the app, go to settings and be sure notifications is still checked. It's listed first under General.