Macintosh HD not available using Option key
I have a 2017 iMac running High Sierra and Windows 10 in Bootcamp.
I have always been able to use the option key to select the Macintosh HD at bootup. However, this option has disappeared, and the screen offers only Windows and the Recovery Drive.
I can restart in the Mac OS using the bootcamp utility in Windows.
Your advice would be greatly appreciated!
iMac, macOS High Sierra (10.13.2), Late 2017 27 inch Retina display