What should I do if my amended return has still not been processed since last year because I didn't mail in my original return first? Can I re-file my original return?

You will need to contact the IRS for information. It might be easiest to set up an account with them.

IRS Individual Online Account  

• Taxpayers with a Social Security number or an Individual Taxpayer Identification Number can create or access their IRS Individual Online Account to get information they need to file their return.
• With an IRS Individual Online Account, people can:
• View balance owed, payment history and schedule payments.
• Cancel scheduled payments.
• Get transcripts.
• View or create payment plans.
• See digital copies of some IRS notices.
• View key data from their most recently filed tax return, including adjusted gross income.
• Validate bank accounts and save multiple accounts, eliminating the need to re-enter bank account information every time they make a payment.
• View, approve and electronically sign power of attorney and tax information authorizations from their tax professional. 

DId IRS send your amendment back to you with reason: no original 1040 on file."  ?"

That's what should have happened.

@kenedyrhode10 

If IRS does not have your tax return, you have to file on paper via US Mail.

Don't forget to sign it.

@kenedyrhode10