The issue you describe appears to be either a software or hardware-related problem. Below you'll find 2 solutions to help determine what type of issue it is and how to resolve it.
A. Software: corruption of one or more system files within the BlackBerry OS. A forced update should restore your BB to working condition, but there is the risk that you may lose your data. If that is a problem, do not proceed.
If you wish to restore the BB, please continue below after verifying that you have the following:
A. Computer (Windows)
B. BlackBerry Desktop Manager software (BlackBerry.com)
C. Latest version of the BlackBerry OS for your make/model BB
To get the OS, simply logon to your carriers website and download the latest version of the OS for your BB. Once the file is saved to your computer, you need to install the setup files just like you would anyother program. When completed, proceed to step #1.
1. Open BB Desktop Manager
2. Remove the battery cover, battery and microSD card (if you have one) from your BlackBerry.
3. Connect the BB to your computer via USB cable (without the battery in place, just the phone's shell).
4. There should be a few drivers that will load automatically and the BB DesktopManager will recognize your BB.
5. Close the BB Desktop Manager once it has been detected.
6. Go to the following path. C:\Program Files\Common Files\Research In Motion\AppLoader
7. Double-click the "AppLoader.exe" application
8. Once it loads, the application will detect your BB and proceed to update your BlackBerry's OS.
9. If you are prompted to backup your BB data, please do so. If not, then proceed to step #10.
10. Select wipe (or secure wipe) to completely remove the existing OS and data.
A fresh, updated version of the BlackBerry OS will be installed on your BB. The process takes about 35-90 mins to complete. Once done, the BB will automatically reboot and if you had an existing backup or created one before wiping/installing, you can now reopen the BB Desktop Manager and restore your backup to the BB.
B. Hardware: The keyboard component has lost its tactile feedback and become faulty or the logic board (what stores the information and interconnect for the BB device) has become faulty and no longer responds to key presses).
In the event of a hardware malfunction, the solution would be to replace the failed component. A replacement keyboard costs $15-45 depending on make/model and can be replaced simply by removing the old keyboard and swapping it out for the new one. A logic board replacement is more expensive around $135-$200 because of the nature of the device. It is the central interconnect or "heart" of any mobile device/computer, as such, it is essentially viewed as buy a new device due to the cost incurred.