My MacBook will only work if it is plugged in. The light on the plug is green and the battery icon at the top of the screen shows a "X". I have replaced the battery with a known good battery but still have the same result...I have to plug it in. Thanks to all for your suggestions. I will probably take it to a Mac store.

I ended up taking my MacBook to the Apple store at the Millena Mall in Orlando and I am really glad I did. I initially tried two Mac repair companies that I found on line. The both told me that the problam was probably in my logic board and that it could cost around $500 to repair. To make a long story short and with a happy ending, The Apple store fixed it for $ 125. :a040: