Did this happen on iOS? Do not uninstall the app -- your missing files are still in the application cache and deleting the app will also delete files you have not yet synced. Please backup your device, and then try the advice on this page to try and recover your original images:
If you are not able to recover files, do you have a recent back up of your device from a time when Lightroom still had yuour images? If so, restore that backup onto your device and then try the help steps again from the link above.