According to the Guardian

Actor Mathew Horne collapsed on stage during a matinee performance of Entertaining Mr Sloane in London today.

Fellow actor Simon Paisley Day called for doctors in the house at Whitehall’s Trafalgar Studios as Horne lay prone for 10 minutes before paramedics arrived, reports suggested.

UPDATE

According to the BBC

Horne fell to the floor in the middle of a scene where he lay for 10 minutes before paramedics arrived to take him to hospital. He was later released.

His spokesman said he collapsed after working flat out for three months.