Do you have an external keyboard and mouse that you can attach, and do they show any response?
Try rebooting the system (hold the power button to hard-reset it, if needed), which should spur the input devices working again. However, if not, then try again rebooting but this time hold the Shift key immediately when you hear the boot chimes, and continue to hold it. You should see a gray progress bar appear indiciating the system is booting to Safe Mode, and then in this mode does the keyboard work properly?
If the system does not boot to the progress bar, then try holding the Command-R keys together at startup (again, at the boot chimes). This should load the Recovery tools if successful.
Along with these, try rebooting with the Option key held, and you should see either a boot menu with your available boot drives, or a password prompt. If neither of these show up and the prior efforts do not work, then your system is experiencing a hardware malfunction and you will need to take it in for servicing.