As has been indicated above, it's a malware attack.
In the vast majority of cases simply running the free version of Malwarebytes fixes the problem:
https://www.malwarebytes.org/antimalware/
NB: When installing it take care to deselect the pre-ticked option to take a free trial of the Premium version. When malware is found use the 'quarantine' option to zap it all.