- What antivirus program are you using?
- Anything remarkable in its log files?
When my Avast Free this week immediately after upgrading Windows to version 1903 did run into trouble and even its own repair option couldn't repair it, I had to uninstall and reinstall it. That was easy and fast.
But it doesn't mean at all that a comparable action will work for you.
On my laptop I am told that virus protection is off. So I open up Windows Security and go to Virus and Threat Protection. There it says that "Threat Service has stopped. Restart it now." So I hit the Restart Now button and it shows that the service is restarting. But after awhile I get the message that it has failed and I should try again later. But I can never get it to restart. What is my problem and how do I fix it ?

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