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You have to just love Microsoft don't you? or maybe not! It seems that they have done it again yes they have went and broke Windows Defender in Windows 10 with a recent update. The update I quote "automatic update, known as Security Intelligence 1.313.1638.0, which was only recently released by Microsoft, is causing Windows Defender to show error messages saying “Threat service has stopped. Restart now,” and “Unexpected error. Sorry, we ran into a problem. Please try again.”
The update apparently prevents Windows Defender to scan Windows 10 for viruses and other security issues. So if you're using Windows Defender to keep you safe then you will find that you won't be able to scan for issues.

It is also not a simple removal of the update to fix this but do a System Restore to a date before the update was applied. If you know the date of the update was installed you can do this by going to updates and checking for the above mentioned Security Intelligence and check for any System Restore before today's date the 16th April this should fix the error and keep you safe again (till the next time)

Just thought I would pass this on to all that use Windows Defender rather than any other AV software.

Thanks BF, good to know.

Thanks BF - you wouldn't happen to know the associated KB would you please?

Hi Colin sorry about the late reply. Some users have reported that the following KB4549951is the culprit but it is more related to the update called Security Intelligence 1.313.1638.0 which is basically the virus definitions. This can be fixed by doing a rollback or system restore to the 16th of April or before. There is most likely a new update to fix this by this reply to you. I am not at my PC as replying by my Cell / Mobile at the moment. Well again sorry for the delay.

Hey BC - absolutely no apology needed mate - thanks for the update.

I was able to postpone installing KB4549951. I think for a few weeks. That would have them sort it, you might think? Thanks BaconFries.

I had the problem yesterday that BF identified, but things are back to normal today. Looks as though MS came up with a quick fix.

Brian

lummis is correct I ran a check at 19:15pm tonight and no errors. To confirm I checked for updates and saw that MS had indeed put out a fix. Now let's hope they don't break anything else....but it is Microsoft so only time will tell.