The thumbnail you are seeing maybe a cached thumbnail from your previous session.

Did you try "placing" the corrupt psd into a new one?

You open a new file with same size or larger and from the file menu choose "place".

You may be able to open the file with any other program that read's psd files, however, they may skip any corrupt layers and load only some of them.

Finally, some psd repair tools can be found online, but I don't know how good they are, and what the limitations of a trial version may be.

 

Yesterday's the past, tomorrow's the future, but today is a gift. That's why it's called the present.