Blackberry smartphone owner could face problems whenever they installed 3rd-party application or theme, their Blackberry will end up to hang or restart continuously. And when you want to undo what you've done, you can't uninstalled that themes or application.
To fixed this problem you can start or "boot BlackBerry in Safe Mode", this mode will enable the BlackBerry to start without loading any 3rd party applications or themes.
How to Start BlackBerry in Safe Mode:
- Do a hard reset by performing battery pull and put it back again.
- When LED indicator light become red, press and hold the Escape button (The Back button in the right of the trackball or trackpad )

- A Confirmation dialog will show in your screen, press OK to enter Safe Mode
- You will notice a "Safe Mode" text on the Home Screen that indicating the BlackBerry has successfuly booting in safe mode.
- And now all you have to do is uninstall the program or the theme that made your Blackberry freeze(or you can choose the program / theme that yout last install)
To exit Safe mode, all you have to do is hard resetting (pull battery) your BlackBerry.
This steps can be done in BlackBerry Curve 9300, Bold 9700, Torch 9800, Bold 9780, Pearl 3G, Bold 9900, etc.
Sources: BlackBerry Technical Solution Center