I just received a klarus xt11 that I bought as combo with an ultrafire dual bay travel charger.after reading some of the opinions of other forum members I was skeptical of leaving this charger plugged in while I ran to the store. So I unplugged from the wall ran to the store a forget to plug it back in. so the battery say over night on the charger unplugged.well I went to check its voltage this morning and dmm shows zero volts and my flashlight confirms this. Is it possible I just ruined this battery?
It IS strongly advisable NOT to leave Li-Ion cells charging while unattended, as you seem to already be aware. Things can go wrong, it is rare, however by always keeping an eye out while cells are being charged, a potential nasty "incident" can likely be avoided, or at least be caught, before the damage is too great.
Also, along the same lines, it is not advisable to leave cells in a charger that is not plugged in. Most chargers will drain cells, to some degree when in the "off" state, some faster than others. This is apparently what happened in your case.
If the cell is a "protected" cell, the cell was probably discharged to the point where the protection circuit tripped, due to low voltage. In most cases, if the cell is returned to the charger when it's plugged in, the protection circuit will "reset", and the cell can then be recharged.
If the cell lacks a protection circuit, the cell has been over discharged and should be discarded/recycled. While it may be possible to recharge an unprotected cell that has been over discharged, damage has likely occurred within the cell. This damage will affect the performance of the cell and additionally, can render the cell a safety hazard. Either way, it's best to discard/recycle unprotected cells that have been over discharged.
My guess is that your cell is a protected cell. Most unprotected cells, even if over discharged, will read some minimal voltage. When a protection circuit trips, cell voltage will read zero.