I am not an expert, by any stretch of the imagination, but it may be that your battery was not sufficiently charged when you disconnected the ac lead to keep your laptop running. Was it updating anything when you disconnected it?
You could try leaving it on ac power to try and charge the battery for a while, then try starting it up again. See if it will then boot up. If it was updating anything when it happened you may have more serious problems and possibly need to recover the system from either a recovery partition or disc.
Perhaps somebody else could offer further ideas.
