I've disabled m2eclipse plugin for a project by clicking on "Disable Maven Nature" as shown in the Figure below. Then the Maven entry disappeared from the menu and I'm unable to restore it.

How do I restore/reset/re-enable so that the entry appears again?
PS 1: I'd like to do that without re-importing the project.
PS 2: I've looked in ${WORKSPACE}/.metadata\.plugins\org.eclipse.m2e.core.ui and ${WORKSPACE}/.metadata\.plugins\org.eclipse.m2e.core but nothing seems related.