Hi. We have a ThinkPad Tablet 2 that would not boot at all. All of the above suggestions was tried without success.
So we removed the back of the tablet and removed the cable from the internal battery - just for a few seconds. Then reconnected the cable to the battery and connected the charger cable to the tablet - and then it began to charge :-)
Apparently there is a protection circuit in the battery that must be reset if the battery is completely drained.
Hope that can help somone.