I had the wrong shutter speed when taking photos on a very sunny day and they turned out to be completely white, is it possible to get the image back in photoshop? Or is it a lost cause?
You can recover "something" in PS if there are differences between pixels (some are completely white, some are very nearly white, some are a bit less white...).
However if the whole photo is white you likely have large areas of totally white pixels from which you cannot recover anything.
Checking your first few shots for correct exposure and color balance is a good habit.
It is a lost cause. You might be able to recover some details, if you are lucky. However, any creativity you put into taking the photograph will most likely be lost. If you took a photograph as a record of something, to capture a particular act or something, you could try to recover some details and it could be enough to serve your purpose but it is still a long shot.
You can always give it a try and see. If it works, it works! else, it doesn't. Keep your expectations low before attempting the recovery.