The IRS says I am a non filer for 2019. That is simply not true. I filed. I am due a refund, and I owed for my state. I need proof I filed. How can I get it?

I have since already paid my 2019 tax bill for my state income tax. The IRS requested additional paperwork in regards to healthcare. So, I KNOW they got my return. I sent that in back in March. Now, all of a sudden, according to them, I never filed in the first place.