How do I get the APK file for an Android app?

An app on Android suddenly disappeared. I didn't uninstall it and it wasn't malicious. The APK file is not getting installed again. What should I do?

• It's weird. So, it was a book reader which wasn't installed from google play but from a website. It was not malicious and it worked nicely for 6-7 months. But few days ago, it wasn't there on the app screen, it suddenly disappeared. I didn't uninstall it. I discovered that its shortcut which I had on the home screen was still there. I tapped on it and it showed a message that the app is not installed. I searched for in Settings > Apps but it wasn't there. I decided to install it again. So I installed the apk file but it was not getting installed. So next, I tried to installed a different apk file but the results were same, apk not installed. What exactly happened and what can I do now, what is the solution? Did that app get corrupt? P.S : Android version is 4.2.1, phone is not rooted and there is enough memory for the app to get installed.

• Answer:

  Did you happen to update your phone? Meaning not just the Android update but the general Android utilities from the Play Store that pops up on your phone? If yes then that could be the reason for disappearance of the specific application. Google downplays applications installed from third party stores. Some of its updates do over write or remove the third party applications without issuing a warning since we give it explicit permissions to do so. Do check if that is the case. Since I am pretty sure I faced a very similar fate with a third party application I used.