Here's some basic things to try (forgive me if they seem simple, but that's usually what gets you):
1. Do a reboot (especially if it's a Windows system).
2. Confirm that the computer's USB port you're using works with some other device.
3. Check both ends of the cable carefully, the iPad connector has to go in properly (not upside-down... the logo on the connector should be facing up).
4. If you have another iDevice cable, try that one.
Hopefully that should cure it.