I have a 4' x 6' x 3/8 " cultured marble sheet in my bath that has pulled away from the wall approximately 1/4 ". How can I reattach?

Was it attached with thinset? Do not try to patch with glue. Best to remove the entire sheet, clean off the wall and the back of the marble. Is this a wet area and what is under the marble: drywall, blue board or cement board?