One of the main area for issues with Windows 10 is the way it handles displays/monitors.

Setup

• Dell E6540 Laptop
• 2 x Dell U2414H connected by HDMI

My monitors were shown in device manager on my Dell e6540. For my dell U2414H I also updated the drivers instead of using the generic Microsoft ones (not sure htis would of made a difference though)

• Goto the intel HD Graphics Control Panel
• Multiple Displays Tab
• Goto the "Select One or More Active Displays" section
• You will find your missing monitor listed and set to none and save
• Set to match the name of your monitor andsave
• Done

Other fixes that you should try first

• Update drivers
• Update monitor drivers
• Reboot your PC with all the monitors turned on
• If using a laptop on a docking station, undock and then re-dock
• Try a different monitor
• Make sure windows 10 is all upto date
• Run intel driver update utility
• Make sure all drivers are present (ie SM Bus / Chipset drivers)