Before you start - if your sauce is broken because you cooked the egg you cannot fix it. You will need to start over.
If it is broken because it didn't emulsify (get creamy) while you were adding the oil then follow these steps to integrate it.
Start with EITHER one whisked egg yolk OR 1/2 tsp water in a small vessel just large enough to hold your sauce and some room to allow for vigorous whisking.
Start by adding a few DROPS of the broken sauce to the whisked egg yolk or the water. Whisk vigorously. Add a couple more DROPS. Try to whisk continuously. It may be awkward to hold the cup and the whisk and add the broken sauce. Whisk vigorously. You should see from the first few drops added that the sauce is changing from rather transparent broken texture of the broken sauce to a more opaque creamier sauce. It is subtle at first and becomes more obvious as you re-integrate more sauce.
If the sauce is not getting thicker and more opaque after say a tablespoon or so of the broken sauce has been added then start over and whisk more vigorously as you are adding the broken sauce. If it doesn't integrate in the beginning it will not correct itself later.
Keep whisking and adding the sauce slowly - like a 1/2 tsp at a time. Ensure that each addition has been thoroughly incorporated and takes on a creamy texture.
Continue until you have re-incorporated all the broken sauce.