No News on 2020 -- 2020! -- Return. What to do? (corrected)

I filed my 2020 return with an extension last October. 

I was expecting a refund and have not received it.

 

I filed by certified mail and have a receipt that it was delivered. But that's all I know.

 

There has been no refund. The IRS "Where's my refund" has no sign of my return, and when I try to call the IRS it's impossible to get through.

 

What should I do? I'm getting worried that the IRS somehow lost my return. I have no idea what to do next.

Help!!