HELP! Can't log-in – Installation screen appears

Hi
I just tried to view my site and this installation page appeared instead of my homepage (or any other page): https://www.rehab4runners.co.uk/wp-admin/install.php
Also the same when I try to access my dashboard, either directly or by clicking through from my wordpress.com account.
I have and this happen once before and it did resolve itself – but I don’t know how!
Any suggestions?
my site is https://www.rehab4runners.co.uk