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Unfortunately my battery has done the same thing after I left it asleep whilst going on holiday for a week. I looked at the link above and it says that the offer of replacement is now closed as of May 09. Bummer!:(
 
Black x on my battery icon too

Left it for 2 weeks - and now have a black x on the battery icon - computer in effect not recognising the battery ... can't find any way to restore it - seems strange that a battery would go terminal just from being left uncharged.

The Apple support site makes a few suggestions - none of which help - it's listed as a non-solved defect at the MacDefects site...

http://www.appledefects.com/wiki/index.php?title=MacBook

it says

Battery Failure : Battery Does Not Charge and is Not Recognized by MacBook

batteryx.jpg

* Inexplicable battery failure, with the battery not being recognized by the MacBook. The user sees an X through the battery logo and the battery itself refuses to charge.

* Exchanging or replacing battery does not resolve the issue, replacement appears to fix problem, but, problem reoccurrs.

* System profiler reports that battery is present and charged but computer will not run unless connected to the power adapter.

* Some users report problem occurred only after firmware update.

* Apple recommended procedures to reset pram does not fix the problem.
 
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