First try hitting F6. On some devices this key turns the touchpad off/on.
Should that not work, then try...
1. Go into "Control Panel".
2. Click "Hardware and Sound".
3. Now click "Mouse and Touchpad".
4. Now go to "Additional mouse options".
5. The Mouse Properties window will open. In here click on your device.
6. Make sure your device is set to enabled. If it isn't then click "Enable Device".
7. Exit both that screen and "Control Panel".
Also, you might want to check for updates to your drivers, including the Synaptics driver (if your device uses one).
If this does not help, and an external mouse works fine, then it may well be a hardware issue.