There are a combination of places where you can disable the mic - so lets take a look at the following:
restart if you haven't already in case this is a glitch (shutting down is not the same as a restart, please make sure you restart)
look for the keyboard key related to the Mic mute function. It is commonly F4, and depending on the computer configuration you may need to hit F4 in tandem with the Fn or Ctrl key to activate the function. When hitting the key you will likely see an onscreen notice showing mute on/off. Older machines have a red led light on the mute key that will illuminate if the mute is on
Windows Start Menu > Settings > System > Sound > Device properties option under input > make sure the disable option is not checked
In that same section of settings use the troubleshoot option to see if that helps
In Chrome on the site where you are having trouble check the site settings. To the left of the web address click on the icon you see typically lock or i button, and then look for the Microphone option or if you don't see that use the site settings and change a permissions settings as desired
There are enough options to go through here it may be easier to schedule a time to do a remote assist so feel free to do that. Of course, if these troubleshooting steps don't work for sure lets schedule a remote assist.