Clear cut thing to do when 18650 Li Ion Cell Has Been Overdischarged
Your cells are very likely irrecoverable, but there is a method that MAY recover them.

Using one cell connect a charger or DC voltage source of up to about 5V via a say 1,000 ohm resistor. This should provide a maximum current of under 5 mA which MAY trickle the battery back into a recoverable state. If this does not work after a day or so then IF you can locate the cell in a location where "vent with flames" / fire / modest explosion is acceptable you could use a 100 ohm resistor - upping max current to under 50 mA.
Neither of these methods is liable to cause "vent with flames" BUT it is possible.

If the cell voltage does rise to 2.5V or more, allow it to rise to say 3V and then attempt charging in the normal manner.  If the voltage does rise above 2.5V then DO NOT leave connected indefinitely to a voltage source above 4V.