I use Windows 10 at work, and for security reasons, I always lock my workstation when I leave the room for any reason. The problem is that my monitor is a Samsung 4K TV and the lock screen puts it to sleep after only 60 seconds, and after a few more minutes, it goes completely off, requiring me to use the remote control to turn it on again. I'm OK with using the remote to turn it on in the morning when I come in, but it's annoying to have to use it after a 10-15 minute conversation in another office.
After several unsuccessful searches and attempts to fix the problem using standard and advanced settings, I finally came across a registry key setting that seems to be required to allow changes to the timeout on the lock screen that puts the screen to sleep after 1 minute.
In case that web site goes away, the key is:
If the value is 1, the UI to alter this setting will not show up in the advanced power settings. If it is 2, the UI will show up. Reportedly, if it is zero, the monitors will never turn off but neither I nor the guy who posted the linked solution has tested that.
The above answer prevents the screen from going black, but preventing the system from sleeping is a different thing. There is another registry entry to prevent falling asleep.
Set the value to 2 and under advanced power settings you now get Sleep → System unattended sleep timeout, where you can set the value to 0 minutes. Rebooting your system to view the new option is not required.