For F and J status, a valid travel signature on the I-20 or DS-2019 is required to return to the U.S. after travel abroad. Each signature is valid for one calendar year. If you do not have a valid travel signature, a U.S. immigration official may ask you to go through a "secondary inspection" or may give you Form I-515 to be completed and mailed to another immigration office. Consult your international advisor if you are given Form I-515. While this is an inconvenience, it is not a disaster. You may miss your connecting flight, but you will still be permitted to enter the United States.