You do not say which browser you are using - it may be a recent update that has caused this -there are quite a few posts about this happening in Chrome as it self-updates. I believe that Google has said that it is working on a patch.
You could try to empty your browser cache, close the browser and restart your PC but if it is Chrome I think that the problem will re-occur.
Many posters have had to use another browser for the time being or roll back to an archived version of Chrome. The problem then is that it auto-updates and the problem re-occurs so another programme has to run to prevent these self updates.