Acrobat Reader opens process but displays no window

often the processes correspond to browser windows when you open a PDF in the browser. close the browser and the reader instances will go away.

which version of reader? try reinstall/repair or install a newer version.

This may be a Protected Mode error caused by an update resetting your  security settings for Reader.

Try editing the system registry

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options

Delete the key "AcroRd32.exe"

Appears to be affecting PDF's opened from Outlook.  Note that the preview handler does work, but if I double click PDF attachments in Outlook they don't open in a window, although I can see processes running for them.  IF there are no reader processes running, I can then launch the main reader program from its icon and open docs inside there.  Also, IF there are no Acrobat processes running, I can open PDF docs by double clicking their icons on the desktop.  Reader version is 2019.021.20061 and says no updates available.

Deleting the acrord32.exe key as above seems to have no affect.

when you double click on a pdf docuement within the file system, explorer window which application opens?
Depending on the outlook option, not all can preview pdf documents from attachment.

Thanks - turning off protected mode worked.
I'm wondering why this suddenly changed as everything was working last week.
Update to Reader?
I wonder about the safety of continuing without protected mode, but for now I'm at least working again.

Either Reader or Outlook, this won't just be affecting you.  Occasionally Office and Reader updates get out of sync and Outlook sees PDFs as a potential threat, which theoretically is correct, so just with Outlook attachments treat with additional suspicion until Microsoft and Adobe are playing happily together again.