Sounds like the file has been corrupted. Are you getting any errors?
If you have a Mac with the OSX version of Numbers, you could try downloading it from iCloud.com and see if you can open it there. If there is no sensitive information in it feel free to PM me with it attached and I'll take a look.
You can also try these more general solutions. They won't hurt anything, and there's a chance (slim for this problem) that they may help.
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When you have problems with an app it's a good idea to close it.
Close: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.