Complex sentence: The painting attracted much attention and was unanimously praised by critics, however, during the 1950's the painting disappeared after having been hidden behind a new ceiling for a renovation and having had a ventilation flap installed within it.
Split into simpler sentences: The painting attracted much attention and was unanimously praised by critics. During the 1950's, however, the painting disappeared after having been hidden behind a new ceiling for a renovation and having had a ventilation flap installed within it.