Quote from ALVR Discord.

If you’re using lighthouse devices and WiVRn/Monado is suddenly failing to connect/start, chances are your SteamVR has updated to an incompatible version.

To fix Set SteamVR version to previous via beta selection

To prevent it from updating without you knowing Set SteamVR’s auto-update behavior to only update before launching (right-click / properties in Steam library). This way you’ll always know when it updates.