What, if anything, may have happened today: some driver update, OS update, new application installed, configuration change, etc..?
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Update your post to include full system hardware specs and OS information.
Make and model monitor(s)?
Explain more about "stuck in power saving mode".
How are the monitors and/or the host system configured with respect to power saving ?
What actually happens? Does the first monitor simply stay black? Any pop-up errors or other messages?
Are you able to get that first monitor to eventually get out of being "stuck" and, if so, how?