Suddenly, running the Macrium app in Windows 10 shows no dashboard, no anything.  I get nothing on the screen, but Task Manager reports that Macrium Reflect Disk Imaging is running.  Help!

Reboot?  If it still persists, you can uninstall and reinstall.  The default uninstallation behavior is to KEEP everything you'd want to keep if you were planning to reinstall, such as settings, definition files, schedules, logs, etc. -- so as long as rebooting the system in question isn't too much of an inconvenience, it's a pretty painless process.

+1 the above.  I'd try the same things.  First, rebooting if you haven't already and if it's not inconvenient or out of the question.  Reboots solve a lot of Reflect problems.  If rebooting doesn't work, I'd then try uninstalling and reinstalling the software.  If that doesn't work, I'd try restoring a known good Reflect backup where Reflect was working and see if that helps.  If it doesn't, then there's probably a hardware issue somewhere.

The problem had to do with something in the settings (changed without human assistance) which caused the program window to load off-monitor.  I have two monitors.  I messed with them a bit (disabling one, then the other in sequence) and got the window to appear.  I then made it go full screen (maximize).  Then by moving the window, I got it to appear properly even when not in full screen mode  I think having two monitors sometime messes with the windows starting positions.  It was easier to five when programs uses .ini files to set their starting positions. 

There have been a few issues with Windows 10 and 11 regarding the use of multiple monitors.  Particularly on Windows 11, as it seems to have a worse time handling that situation than Windows 10 did.