Sorry that images in Lightroom app on your mobile have stopped syncing. That's definitely shouldn't have happened. Let us help make it right. Please refer the steps below:
Ensure that your Lightroom for mobile app is up to date. To keep your Lightroom for mobile (iOS) app up to date, see Apple's documentation Turn on Automatic Downloads. To keep your Lightroom for mobile (Android) app up to date, see Google's documentation Update downloaded apps.
Ensure that your are running Lightroom on your desktop and Lightroom for mobile app on the same Internet WiFi network. Syncing is paused over the cellular network, if you've enabled Sync Only Over WiFi in Lightroom app's settings. To know more about app settings and preferences, see Lightroom for mobile (iOS)|Sidebar and Lightroom for mobile (Android)|Sidebar.
Ensure that Lightroom for mobile is running in the foreground, especially when the syncing is in progress. To achieve this, make sure that your mobile device does not go into the sleep mode.