Is there a way to restore a field (in views) which has been removed using 'this block (override)'
You can basically revert the fields on the sub-view back to the master settings and list by:

-- clicking on the down arrow on the top right of the FIELDS section.

-- In the popup, change the For dropdown value to Revert to Default. This should effectively bring back the master list of fields, assuming that's where your missing field is.

-- any customisation for this specific view's fields must be re-done.

This same process can be done on any of the panes on the view's edit page - /admin/structure/views/view/my-view - if you want to revert anything.