How can I fix a hollandaise sauce after it has split?
You can rescue it by starting the process again, with an egg yolk in a bowl over a bain-marie which you whisk until it starts to thicken a little.  then incorporate a little warm melted butter into the egg yolk.  Once this is incorporated you can slowly start to add the split hollandaise and this should then incorporate into the new base, unsplit.

There is a video here