DIAGNOSIS The camera is stuck in its setup mode because it lost its Wi-Fi connection or the network credentials changed. That solid white light is its 'ready to connect' indicator.
FIX You need to force it back into setup. In the Ring app, go to your Spotlight Cam Pro in Devices, tap Device Health, and tap 'Reconnect to Wifi or Change Network'. Follow the prompts. Usually, you'll need to hold the setup button on the camera for about 15 seconds until the light starts to pulse, then let go and complete the setup in the app.
IF THAT DIDN'T WORK Try a full device reset. Hold the setup button for 30 seconds. The light will flash a few times and then go off. Wait a minute, then press the button again. It should start pulsing white, and you can set it up like a brand new device in the Ring app.
HEADS UP After a reset, you'll have to redo all your motion zones and linked light settings, which is a pain. It's worth double-checking your router's 2.4GHz band is active and you're using the correct password before you reset.