I'm sure someone else has had this issue when hooking up external monitors to the Steam Deck. Change the power state and all of a sudden the monitor no longer turns on. Turns out the user settings is likely messed up.
Search for "Display Configuration" and check if the missing monitor appears under the "Device" drop-down, or try mouse gesture swiping to reach the missing monitor. Click the "Enable" checkbox to activate it again. I do not know how to keep it enabled when changing the Deck's power state.
