Part of Netflix's appeal is that it's so easy to use and available on so many devices. If you are experiencing problems with streaming movies or TV shows on your iPad, troubleshooting the issue isn't as difficult as it can be with other, more complex apps.
The most likely explanation is that the iPad does not have a strong enough Wi-Fi or data connection to stream video successfully (Netflix recommends a minimum broadband connection speed of 0.5 Megabits-per-second). Move to an area with better Wi-Fi or mobile reception to see if this fixes the issue. If you're connecting via Wi-Fi, test the connection with another device (such as a smartphone or laptop) to see if you can access the Internet normally.
It's possible that a problem with the app -- such as a software glitch or temporary bug in the cache -- is preventing it from operating normally. Tap and hold the app's icon on the iPad home screen, then tap the cross and choose "Delete" to remove the app from your device. Reinstalling Netflix from the App Store should reset its configuration settings and resolve any issues related to the app itself. The process also ensures you're running the most recent version of Netflix for your iPad.
Check you are using the correct username and password to log into the Netflix app (you can sign out using the button at the bottom of the main Netflix screen). Check your account is not in use on any other devices by visiting the recent activity page in a Web browser -- you can only have one or two streams running simultaneously from the same account. Close the Netflix browser window or app on any other computers and devices where you are signed in. It's also possible that there's a technical problem with the Netflix service itself -- check technology news sites and the Netflix Twitter account for reports of any outages.
Another alternative is that a problem with the iPad itself is preventing Netflix from streaming content. Open the Settings app and tap "General" then "Date & Time" to make sure the "Set Automatically" option is selected. If this isn't functioning correctly for whatever reason, turn it off to set the correct date and time manually. If the date and time reported by your iPad is inaccurate, this will prevent Netflix from working correctly. Resetting your iPad can also help fix a variety of problems by closing down all open apps and rebooting the device -- to do this press and hold the Sleep/Wake button and Home button until the Apple logo appears. When the home screen returns, launch Netflix to see if the issue is resolved.