You can perform a soft reset by simply using the Reset pinhole button on the bottom of the device. If that doesn’t solve the problem, you may need to perform a hard reset instead.
Performing these steps will wipe all data off the device. You will be able to resync any data synced to your computer though.
1. Turn off the phone.
2. Press and hold Volume Down and Enter, then press the Power key.
3. Release Power, and continue pressing the Volume Down and Enter keys until you see a message about data being deleted.
4. Release Volume Down and Enter.
5. Press Volume Up to perform the hard reset.
Doing these steps may solve issues with thh HTC Touch Diamond freezing or becoming unresponsive.