If you are unable to connect to iTunes here are some things to try:

1. Make sure your iOS operating system is up to date (Settings > General > Software Update).
2. Make sure your date, time and time zone are correct on your device (Settings > General > Date & Time).
3. Check your device restrictions (Settings > General > Restrictions).
4. Check and verify that you're in range of a WiFi router or base station. If you're on a device with cellular service, make sure that cellular data is turned on.
5. Try resetting (turn off and on) your wireless router.

You could also try these steps:

1. Sign out of your ID (Settings > iTunes & App Store).
2. Reset your device's wireless settings (Settings > General > Reset > Reset Network Settings).
3. Reconfigure wireless settings on your device.
4. Without signing in, try and make a purchase (it should prompt you to sign in again at this point).

Alternatively, try this:

1. Go to 'Settings'.
2. Scroll down to 'Safari'.
3. Scroll Down and tap on 'Clear Cookies and Data'.
4. Tap on 'Clear Cookies and Data'.

If you are still unable to make a purchase or connect, please see this Apple Support article on declined payments. You might have to contact Apple as they may have blocked purchases.