I mailed my 2018 Federal Taxes 4/11/20...they have not been processed yet. I was told by IRS to go ahead and file my 2019 taxes...I want to use efile, since mailing didn't work well. There isn't any record of my 2018 AGI and I don't have a pin from last year.
If you did not file a tax return for 2018, you can still file in 2019 by selecting 0 as your prior year AGI. This of course will only work if the 2018 taxes haven't been processed yet, but the good news is you get fairly quick feedback of that information.
You could also request a transcript of your 2018 taxes, in the case that they have accepted and processed the 2018 return and you just don't know it yet. 6 months is a very long time for them to not have processed it yet, so I'd be concerned, and consider re-sending the 2018 taxes in as well.