Bring up the Force Quit panel by pressing option+command+Esc keys. If Pages is on this menu, select it, and press Force Quit. Repeat this to see if Pages is still on the Force Quit panel. If not, just close this Force Quit panel.
Additionally, in System Preferences : Dock panel, select the Show indicators for open applications. This will put a light "dot" beneath any open application in the Dock:

I suggest that you also reboot once in Safe Boot mode, and see if Pages will launch there. Press and hold the shift key when you hear the boot chime — until you see the boot progress bar. A login panel will appear. This particular boot process is slower due to System housekeeping, and there may be some screen antics, and these are normal. When you are signed in, press and hold the shift key while launching Pages. Whether it launched or not, reboot normally. Press and hold the shift key while launching Pages again.
Resolved?