What connection are you currently using from the Mac mini to the Viewsonic?
First try unplugging and then plugging the monitor back into the Mac mini.
If that doesn't help, shut down the Mac mini by holding the power button for 10 seconds, turn off and disconnect the monitor. Then reconnect, turn on the monitor and startup the Mac mini.
If that doesn't help, shut down the Mac mini by holding the power button for 10 seconds and restart the Mac mini with a NVRAM reset, as per > Reset NVRAM on your Mac - Apple Support
If all else fails, try a different connect between the Mac mini and Viewsonic monitor.
the connections options to the monitor include:
HDMI to HDMI
USB-C to VGA
USB-C to HDMI
USB-C to DisplayPort