Pop-ups which warn you that your camera and microphone have been turned on are usually created by malware which wants to trick you into buying totally unnecessary ((and usually fake) security software. i.e. the camera and microphone
haven't[i been turned on - it's just a scammer trying to scare you. (People who're using desktop PCs, that aren't even equipped with webcams or microphones, see those messages too).
If you've [i]already] got Malwarebytes Anti-Malware on your computer, run a scan with it. (I don't recommend it as a fresh download, as it comes with a free trial of the 'Pro' version - which seems to slow some computers down quite a bit).
Otherwise download and run AdwCleaner. (There's no installation process; you simply run the downloaded file directly):
https://www.malwarebytes.com/adwcleaner/