Can I wake up a sleeping Li ion battery pack without dismantling it?
I only have experience with batteries from one supplier, but in that case, when the voltage went too low the circuit was effectively disconnected. If you are seeing 5.6V then the protection circuit hasn't actually cut off access to the cells.

It is possible that the protection circuit isn't preventing charging, but that your charger isn't attempting it.

I would try another charger. If it doesn't work, I'd try to inject a small current into the entire battery pack until the voltage gets high enough for your charger to be happy.