It helps us to know whether you're using a laptop, a mobile phone (Android? iPhone? Windows?), an Android tablet, an iPad or something else. (We're not mind readers!!!).
However, if it's a laptop then it seems suspiciously like some form of ransomware. Does the message appear as soon as you turn the machine on or does it only appear when you open your web browser? (If so, have you got another web browser installed?)