I had a warning page pop up and lock my browser-the warning was from interpol saying if I didn't pay them a fine my computer would remain locked, this is the same sort of thing emanating from cryptlockers.
It relies on javascript to actually invade your computer, this could have been inserted into a reputable site. In my case this was a ticket booking website, generally they themselves have been hijacked without knowing it. Once the javascript is activated on your system it then spreads, all I have done is turn off javascript for browsing purposes. If I go to a site that I trust then I turn the javascript back on, but it is turned off for safety's sake nowadays.