How long have you had this ?
Can you go Start - Settings - Update & Security and double click on Windows Defender to see if it's turned on or off.
One way to determine if an antivirus program is the cause of a problem, is to see if you still get it in Safe Mode with Networking, but Ethernet connect first as Win 10 disables wireless in that mode, although it can be manually re-enabled.
Some have reported this red cross happening following Windows updates.