Look at the reject message contained in the notification. It should tell you the reason for the rejection. If you no longer have the e-mail, you can always check the e-file status of your return by clicking here: Check your e-file status
If you don't see a reason for the rejection, go ahead and transmit the return again. If you get the same rejection a second time and you see no mistakes on your return, select the file by mail option to paper-file your return. Sometimes, returns keep getting rejected even though there's nothing wrong with them (data mismatches, dependents already claimed, and so forth).
In these cases, we recommend paper-filing by mail rather than wasting additional time trying to "fix" something that is beyond your control.