Just happened to me. Accidentally clicked one of those annoying 'please change to edge' buttons when I was distracted. To solve :
In Edge, click the three dots (ellipsis) in the top right hand corner,
then select Settings, then select Default Browser.
There is then a heading "Let Internet Explorer open sites in Microsoft Edge". In my case this had been switched to 'Always' - I switched it to 'Never' even though this isn't the recommended setting.
Then I closed Edge and opened Internet Explorer without issue.
Hope this helps some folks.