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 drivers.
If this does not help, and an external mouse works fine, then it may well be a hardware issue.
Try the F6 key. I think it toggles the touchpad on and off. You probably just hit it by mistake.
My wife's Gateway "suddenly" had an issue with the touchpad. Usually I can find a problem quickly, but I did not know there was an F6 key toggle for that feature. Thank God that I found you!! Was about to spend $500 on a new computer for her.