It could be that you just hit a key to turn off the keyboard and turn on the on screen one. If that is the case, then you would just need to switch it back. If you dn't know what key you hit, you can make the change in the Control Panel, or there should even be an option in your Taskbar.
Now if that isn't the case, and you are just getting nothing, try the following...
NOTE: If you need to type in a password to even get into Windows to do the following, make sure you attached an external keyboard first, to get past the password. Then disconnect the external keyboard and try the following...
1. Uninstall the keyboard driver.
2. Reboot the computer (without reinstalling the driver).
3. When it restarts, it should reinstall the driver on its own.
If this doesn't work, you may wish to try connecting an external keyboard and see if it works fine. If it does, then it could be the keys themselves that are the issue.
You may also want to try updating the Synaptics Driver if the above doesn't help.