Check the following things
Does the employee record have a hire date AFTER the dates on the time sheet?
Does the employee record have a termination date BEFORE the dates on the time sheet?
Is the time period locked?
Is the project inactive?
Does the project have Allow Time checked on the preferences tab?
Is the restrict time to assignees preference checked on the project and the employee not assigned to the task?
Any fields set as mandatory on the time sheet but now hidden on the time sheet record?