In pages, I can no longer highlight a word then click on it to look up the definition. Is there a way to correct this?

Are you using a trackpad?

If so, go to System Preferences > Trackpad and make sure 'Look up & data detectors' is checked.

If you are using a mouse there is probably a similar setting in System Preferences > Mouse (though I don't have a mouse to test that).

SG

You do not need to highlight a word to look it up in the Dictionary. Instead, just pass your pointer over the word and press control+command+d. This is not restricted to Pages. Or a three-finger tap from a trackpad, if configured in the Trackpad preferences pane.