Go into Settings > Mail, Contacts, Calendars and make sure Fetch New Data is set to Push. If it is, tap on the line to expand your choices and then Select Advanced. Under Advanced you'll see each email account. Make sure your account is set to Push here also.
Not all account types support Push. If your's does not you may have been using Fetch instead. In that case make sure the account is set to Fetch under the Advanced area, then go back the screen that lets you turn Push ON/OFF and make sure you've got Fetch set for something other than Manual. The more frequently you set Fetch for, the more battery it will take, so if you don't need immediate notification choose the longer time periods.
Occasionally Push notifications can be delayed if there is server overload or problems; so don't be surprised if it just magically goes away.
If all this is set correctly and it's still not working, try these usual fixes:
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Close/Clear the app: 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. Apple recommends you restart the iPad at least once a month.
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.
