SOLVED Called MSI Support, reset cmos by shorting JBAT1 with a screwdriver for 20 seconds with power connected but switched off.
Ryzen 5 1400 MSI B350 Gaming Plus Corsair Vengeance LPX 2133 (2x 8gb) MSI Nvidia GTX 1060 3gb Samsung Evo 500gb SSD
Assembled neatly into case, turns on, fans all run. No signal to monitor. 4 long beeps.
Tried re-seating everything, including all hookups; tried removing CMOS battery for a few minutes (with power plugged in and computer off, per instructkons). Same results.