IF you explain what happens when you try to download apps, it might help, but in the absence of any further information from you, try signing out of your ID, restart your iPad and then sign in again.
Settings>iTunes & App Store>Apple ID. Sign out. Restart the iPad by holding down on the power button until the slider bar appears, then slide to the right to shut down. Hold the power button until the Apple logo appears to power up again. Go back to the settings as described above and sign in again. Try to download apps. If that doesn't work, post back with your results and provide details.