Quote from LVRA 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.