iMac won't completely start-up

iMac (Retina 4K, 21.5-inch, Late 2015) running macOS 10.13.2. After a recent Microsoft software update and restart, the iMac would not complete the start-up sequence. The white horizontal scroll bar below the Apple logo, comes to a stop at the very end of its path. I've waited 10 or 15 minutes for the scroll to end and the start-up to complete, but no luck. I restarted the iMac (using its internal HD), but the incomplete start-up continues. Doing a start-up from my external back-up drive with a SuperDuper! back-up file works fine.