It's quite easy to accidentally disable the touchpad on a HP Probook 4520s: double-tapping the left-top corner of the touchpad triggers the enable/disable switch. A LED indicates whether the touchpad is on or off: when the LED is lit, it means the touchpad is disabled, and vice-versa.
Users who are not aware of this feature can be looking to re-enable the touchpad for quite some time. A customer had a similar issue, and I was unaware the touchpad switch was actually built in the touchpad itself.
To re-enable the touchpad, all he had to do was double-tap the LED in the left-top corner. From a design perspective, it's quite neat, but not really user-friendly, or do we really have to read the manual that no-one ever reads...?