I checked through the EventVwr and found msi installer messages complaining about .Net Core 5..00 missing. I installed the .Net Core 5.0.16 runtime and now SupportAssist works.  
SupportAssist has been working OK on my PC since January but I haven't been able to get it to start in the last 2 weeks.  see the screenshots below. Is there a log file anywhere that might help me find what the problem is? Win10 Pro, XPS8950   These are the 4 stages it goes through [IMAGE][IMAGE]second message[IMAGE]third message[IMAGE]fourth and final message
SupportAssist has stopped working