Redirecting blog elsewhere

Hi
I’ve made a pretty fundamental error today: in backing up and moving my WordPress installation elsewhere I have corrupted my original blog. I had hoped to have the two running in parallel during a handover period. However, now what I want to do is put up a simple message with the new URL in. When users connect to https://eharrin.free.fr/wordpress, what is the file they (are supposed to) see? I imagine that is the one I will need to change, just adding a line to say that it has moved. I have tried to do this with index.php but it made no difference.
Any help would be gratefully received.
Thanks