First try restarting the device. It may just be a glitch that it isn't listed. If that should not resolve it you can try downloading a file manager app from Google Play. However, I am not sure the original phone app will be there.
If you really wanted the original app(s) on the phone, the only way I know for certain that you could get it would be a "Hard Reset". However that would wipe the device, so you would have to back up all your person files elsewhere (google, cloud account, computer, etc.) and you would have to reinstall all personal apps after the reset. Should you wish to do that, the instructions on how are below.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
6. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
7. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
8. Then press the "Power" button to hard reset the phone.
9. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.