The PCA released a statement at the request of Thorpe's family, which read: "Graham Thorpe has recently fallen seriously ill and is currently in hospital receiving treatment.
"His prognosis is unclear at this stage and we ask for privacy for him and his family at this time. Our thoughts are with Graham and his family."
Former Surrey left-hander Thorpe was renowned as one of the finest English players of his generation, played exactly 100 Tests and scoring 16 centuries before retiring in 2005.
He went on to begin a coaching career in Australia, where he worked with the likes of Steve Smith and David Warner at New South Wales, before joining the England and Wales Cricket Board as a batting coach.
He worked as assistant with the senior side under Trevor Bayliss and Chris Silverwood, stepping up to lead the team in this winter's Sydney Test against Australia due to Silverwood's coronavirus diagnosis.
He took the Afghanistan job in March.
Surrey sent their thoughts to a "favourite son" of the club, writing on Twitter: "The thoughts of everyone at the club are with Graham, his wife Amanda, and family at this time.
"He is an icon of the English game, known by millions of cricket fans and forever a favourite son of Surrey CCC."