You can unlock a keyboard on a PC by clicking Start>Control Panel>Accessibility Options>Keyboard Tab, then uncheck all the keyboard options. Restart the computer again after this process, and your keyboard will be unblocked.
A keyboard will be blocked if you hold the SHIFT key for more than 8 seconds. This action activates the "sticky key" feature which results in the keyboard remaining in a frozen state until unlocked. In this case, the above procedure will unlock your keyboard. In case the keyboard loses connection with the computer, shut down your PC, and unplug then re-plug your keyboard to re-establish the connection.