Student class can't see assignment
If it's for the whole of the class, check to make sure that
the assignment is actually published (look for timestamp underneath published in upper right hand side) AND
the assignment start/due date is appropriate.
If both of these look good next step is to masq as the student and see if the assignment is missing from within the student's class. If missing, generate missing cua (usually for one to a small handful of students, not a full class).
masq as instructor
drill into specific assignment
edit settings
check that start date is appropriate
if students complaining, start probably in the past
sometimes wrong year/month is put in here
If the assignment is still active check to see if late submits are needed for the students effected and create them