It could be the AC adapter for your laptop but, more often than not, it's a loose center pin on the jack that the cord plugs into on the computer. This is a pretty common problem-- I see it a couple of times a week at our shop. That center pin can take a beating from the cord getting pulled around and just general plugging/unplugging of the cord into the computer.
Get a volt meter and check the power output from your adapter and if it's outputting the right current, then chances are the problem's with the jack on the laptop.
Any competent computer repair shop should be able to fix it with relatively little expense. If that's all it is, it's one solder joint that needs repaired, or at most about a $15 part that needs replaced.