Why is my touch screen unresponsive?

The first thing that you will want to do is of course physically check your touch screen for any signs of damage. Check to make sure that it is clean, and that there's no dirt or oils that could be interfering with touch. Also check that there's no apparent water damage.

You can perform a hard reboot which is an easy but helpful method. This can fix a wide variety of issues.

You can also try a factory reset. Before you reset, make sure to back up your data either using iTunes, iCloud, or a third-party app

If it is a hardware issue, you will have to contact Apple and possibly have your screen replaced.