unreachable browser exception
Reason may be any of the following:
1) Maybe the browser that your script is pointing out doesn’t exist.
2) You have multiple windows and you need to pass the control to the specified window by using driver.switchTo() method.
3) Your internet speed is very low or your internet connection has been disconnected.
4) The link that your script had clicked may have opened new window and the control is still on current window. In that case you get Unreachable browser exception. If try to perform any action on unreachable browser you will get NosuchElement Exception.
Hope my answer helps.