Stopped working after latest 10 update

I've logged a request for help at Parallels Support but meanwhile hoping someone here might help as don't know how long they take to respond!
I have been using Parallels 10 for Mac without a problem till today when foolishly it suggested I install the latest updates - which I did. Now when I start the "virtual window" I get the window but absolutely nothing apart from a nice black space with the usual toolbar at top. There is one alert button top which tells me the Parallels Tools are Out of Date. So I click it and choose Update Parallels Tools but nothing whatsoever happens. What can I try? Do I try and reinstall this Parallels Tools (and if so how?)

 

Very bizarrely I have a solution. I plugged a mouse into a USB socket on the back of the Imac and almost immediately - unless complete coincidence - the Parallels Tools update window appeared and started updating. Then everything fine.