If the Nexus One is freezing or won’t start properly, you may want to try to start it in Safe Mode. Safe Mode will allow you to start the smartphone with the default software running so you have a chance to disable any software you’ve installed that may be causing the problem.
Perform these steps:
1. Power the device off. Take the battery out if it won’t turn off normally.
2. Power the device on.
3. When the logo appears, press and hold the Menu button.
4. Continue holding the Menu button until you see the home screen.
The words Safe Mode should appear in the lower left corner if you performed the steps correctly.