Sefer Torah fell on the floor. What shoud I do?
It is customary for anybody in the presence of a Sefer Torah falling on the floor to fast.
See Magen Avraham 44:5; for a detailed explanation, see Iggros Moshe 3:3; Tzitz Eliezer 5:1.
The Chida adds that there is an actualy obligation to fast when the Sefer Torah drops from one’s hands; the customary fast applies when one is witness to the falling of a Sefer Torah.