We're so sorry to hear about this, could you please let us know which version of Photoshop are you using & which operating system are you working on?
Also, did you try resetting preferences of Photoshop & see if it works fine after that? https://helpx.adobe.com/photoshop/using/preferences.html
If that doesn't work then please check this forum thread & let us know if it helps: Why are colors faded when saving from Photoshop? And other weird things...
It's cc2018 - I will try the other things you suggest and come back to you
That seems to have worked - thanks for your help
Glad we got it working, please feel free to reach out to us for any future queries. Also, would request you to mark the appropriate answer as correct, as it might help other users to find the solution to this issue.