Re-enabling window.alert in Chrome
Close and re-open the tab. That should do the trick.
I can see that this only for actually turning the dialogs back on. But if you are a web dev and you would like to see a way to possibly have some form of notification when these are off...in the case that you are using native alerts/confirms for validation or whatever. Check this solution to detect and notify the user https://stackoverflow.com/a/23697435/1248536
In Chrome Browser go to setting , clear browsing history and then reload the page