If you have already interacted with the file, run a deep scan using a trusted tool like Malwarebytes or Windows Security.
Once clicked, instead of playing a video, the file installs malicious software. This can range from Adware (bombarding you with pop-ups) to more dangerous Spyware or Trojans designed to steal saved passwords and banking information. Sexy Girl (558) mp4
For more information on identifying these types of threats, you can refer to the Federal Trade Commission's guide on malware. If you have already interacted with the file,
Enable "Show file extensions" in your operating system settings. A real video will end in .mp4 ; a virus might hide as .mp4.exe . instead of playing a video