Have you opened it up at all? This is pure conjecture, but this sounds to me like one of the battery cells might have kicked the bucket. If you have a look at the internals, the battery is divided between the left and right sides and immediately near the fans. Battery electrolyte usually smells very sweet. And a bunk battery could definitely keep this from powering up properly. It may not be capable of outputting enough energy to fully boot.
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It definitely sounds like internal inspection is necessary if nothing else.
If you do decide to open, please be very careful. If the battery has leaked it may be dangerous.