It's probably a glitch, so you may want to wait before you try accessing the app again. If you're on Android, try closing the app again and "kill" it by pressing the "Recents" on-screen button on your phone, then swiping your finger upward to properly close the app. Reopen Instagram and you should be able to see the default feed. Creating a habit of "killing" apps after you've used them will also help you conserve your phone's battery life.