Macbook Air 2008 won't boot after error message asking to shut down due to low battery
If you can't boot to a grey screen after resetting the SMC and then connecting a working charger for 15 minutes, you have a hardware issue.

The steps a hardware technician would perform are as follows:


Disconnect everything you can from the Mac
Try a power on when connected to power. (Observing whether the charge LED goes green or orange is an important clue to the health of the SMC/logic board and battery)
Reset the NVRAM and try a boot with the option key held down listening for the blower to start up
Charge for 10 minutes
Reset the SMC again
Now that things you can do externally are basically exhausted, now they would open the Mac case and proceed with observation/testing - perhaps with disconnect the battery and test charging and boot again