If you had an older version of Pages then it was saved and moved to the /Applications/iWork folder when you updated. The new version of Pages.app will be in the /Applications folder. The newer version lost many features when it was updated to v5.0. If you have both versions you'll need to decide which to use. But keep in mind you can't open documents created by the new version with the older version of the app.
Here is a comment from another user with this problem which explains how it was fixed:
"I was also having the same problem and upon speaking with an IT support specialist at Apple Care, they explained to me that my MacBook was trying to open my documents with the old version of Pages, even though I had recently installed the new version along with the Mavericks update. I had to quit the old version of Pages on my dock (right click, Quit), remove it from my dock (right click, Options, Remove from Dock), and using the Finder (Finder, Applications) find and open the new version of Pages (double click). Once open, the icon for the new version of Pages appeared on my dock. Right click, Options, Keep In Dock.
This fixed my problem, allowing my MacBook to open documents with the new version of Pages! :-) I hope this helps anyone who has this problem in the future. I took the initiative and did the same with my Keynote and Numbers apps because for some reason the old versions of those were on my dock too, despite the recent update. IT support rep explained that happens sometimes."