If there is no record of your refund, you have 3 years from the date you filed your return or 2 years after you paid the tax due (whichever is later) to file an amendment. You will have to file a 2015 paper income tax return. You will also get interest on the $672 refund since the IRS has been holding the money.
To file a 2016, 2017 or 2018 income tax return, see How do I file a prior year tax return?
If you lost your refund check, you should initiate a refund trace:
Call the IRS at 800-829-1954 (toll-free) and either use the automated system or speak with an agent.
However, if you filed a married filing jointly return, you can’t initiate a trace using the automated systems. Download and complete the Form 3911, Taxpayer Statement Regarding Refund, or the IRS can issue you a Form 3911 to get the replacement process started.
Your claim for a missing refund is processed one of two ways:
If the check wasn't cashed, you'll receive a replacement check once the original check is canceled.
If the refund check was cashed, the Bureau of the Fiscal Service (BFS) will provide you with a claim package that includes a copy of the cashed check. Follow the instructions for completing the claim package. BFS will review your claim and the signature on the canceled check before determining whether they can issue you a replacement check. The BFS review can take up to six weeks to complete.
[Edited 3-12-2020|10:33 am PST]