If delete-selection-mode wasn't moved or deleted, try
M-: (require 'delsel)<return>
which should load the file providing delete-selection-mode. If this command fails, the "delsel.el" file may have been deleted accidentally (or is no longer provided with emacs). If it succeeds, but delete-selection-mode still fails with an error, look at the file "delsel.el" and see if the mode name has changed.