Why is my screen all white?

For future reference Christelle, if something spills on it (or another device) you should do the following:

Find out what kind of Mac you have - you can use our ID-Your-Mac tool. Once you know that, open it up using the corresponding guide on one of the device pages for the Mac.

Once you're at a step where you can see the logic board / parts where liquid got into, you should use some strong isopropyl alcohol and a soft brush to remove any white-ish residue (corrosion) that you see. Let the device dry <thoroughly> to make sure all liquid is gone (this will likely take a day or two in a warm, dry place if the device is opened up. (Normally, putting a smaller device in a bag of rice can also be useful at this point.) When it is dry, put it back together, and you will have your best chance of it working.