The email that told you it was rejected has the rejection code or rejection message. The most common reason is AGI. But now you cannot e-file anyway. E-filing is now closed, and your return has to be printed, signed and mailed.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.