How to fix laptop that only runs on AC power

You probably have a faulty battery. You can verify this if using Microsoft Windows as follows...

1) Click Start - Control Panel - Performance and Maintenance (If Category View) - Power Options - Power Meter. You should now be looking at the Power Options Properties window.

2) The 'current power source' should be 'AC power' and if your battery were OK the 'Total battery power remaining' should be > 0% and probably charging. If the battery icon is completely grayed out or not there at all then you probably have a sick battery.

3) Check the connectors (usually the metal fins of the laptop itself) and make sure they are not damaged, bent or dirty. If there is no obvious reason for a bad contact then grab something plastic and pointy and slightly (very slightly) bend the metal battery contact fins then gently wiggle the battery back into position and run steps 1 and 2 again.

Hopefully you'll be back in buisness but batteries do deteriorate with age and a replacement maybe what's required. If so be sure to grab the model numbers and details of your old battery and be mindful of the environment in its disposal.