Hardware Causes

• If you iPhone has external damage such as a crack on the screen, this can lead to the keys sticking while you type. Even accumulated dirt or spills on your iPhone can cause problems with the keys. Internal problems may also be to blame. For example, if your iPhone lacks sufficient memory to complete basic tasks, the device may not be able to keep up with your typing, which can cause the keys to stick or freeze.
  
  

Operating System Issues

• Apple periodically releases OS updates for the iPhone. If your iPhone is still running the older, outdated OS, the device may develop glitches, such as the keys sticking or freezing when you open applications. Connect your iPhone to the latest version of iTunes and update your OS through the software. Updating should remove any performance problems related to the OS. If it does not, seek assistance from Apple or your wireless provider.
  
  

Self Repair

• If your iPhone is still responding to other commands, simply restarting the device may alleviate the problem. Try to restart and fully charge the device before taking any additional steps. If this does not fix the issue with the keys, perform a soft reset on your iPhone. A soft reset will restore some of the factory settings but will keep your saved data intact. As a last resort, you can restore your iPhone through iTunes. However, back up your data before restoring as this will erase everything on the device.
  
  

Professional Repair

• If your iPhone has damaged hardware, or does not respond to your self repair attempts, you can have the device professionally repaired. Apple repairs iPhones for a fee. You can also take your iPhone to a third-party repair store, but doing so will void your warranty. Using Apple services to repair your phone will not void your warranty.