I just "inadvertently"  transferred 2018 Form 1040 into a 2019 Form 1040 into which I had already  entered a very substantial amount of 2019 data so now 2019 data seems to have disappeared and I appear to be starting fresh!

Can I undo the transfer and have my 2019 return back?

I just do one return - my own.  What I was trying to do was just transfer an old Schedule E from 2018 to replace a new one I was completing by adding some old data (fully depreciated asset that no longer appeared)