Drained my sample cell to 2.6V accidentally during test. Now my 26650 stopped charging at 3.7V for 30mins. OPUS internal resistance test is 56 which is normal. Is there anything I can do to revive the battery? Will discharge refresh option helps?
Did your charger mistake it for the wrong chemistry (lifepo4) ?
I would think the cell is ok. Iirc going below 2.5v is when it can start to damage the cell.
I did fully charge that particular battery to 4.19V at least 3 times previously. Problem is that it is not completely dead yet 
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Do you mean every time you charge it to 4.2v it doesn’t hold it’s charge?
Discharge - Refresh options……on Li-on?!? NO NO NO NO!
Methinks you need to ‘refresh’ your basics on batteries. http://www.batteryuniversity.com/partone.htm
2.6v was not good for it but it should not have been lethal by any means.
WHAT are you charging it with, and HOW are you charging it (settings)?
26650 is a large battery so it could well appear to ‘get stuck’ at 3.7v for awhile if charged at a low rate.
I tried to recharge immediately after I discovered it discharged to 2.6V. The highest value I got was 3.7V. It did not rise for next 20-30mins... Battery temperature is normal.
Are you charging on the opus BT 3100? Could the switch on the inside to change the voltage have moved?
When I test imr batteries I can go to 2.67V as I usually do to test them… And never got an issue in later recharge cycles. Did with 25R, 6 he2, vappower IMR 26650 5200mAh and efest IMR 4200mAh.
Nothing particular happened… Tested with Reaktor 300… You can check my graphs with voltage, capacity, current and temperature… No problems…
My other batteries charged to 4.2V on other slots. Yes, I am using Opus BT 3100 v2.1.
The only Li-ION that I managed to revive was Sanyo 14500. I seldom discharge my cell below 2.8V deliberately especially unprotected ones. I did experience similar behaviors with low grade China cells (batteries harvest from powerbanks and fake Samsung from DX). I have yet to save any of these problematic cells in the past. Guess I will give it a try with the refresh function on the charger today.
What kind of battery is it? Is this a protected battery?
Have you tried charging ‘that’ battery in the other bays and still got the same result?
Unprotected 26650 from PALIGHT. 
I do not think it is the bay fault. I use the same charger at least 3 times per week.
UPDATE: I finally got it charged to 4.2V using the refresh function. From 3.69V>~2.9V>4.2V took 5hrs 15mins for 1A setting. Capacity value shown has dropped by 200mAh from previous test. Glad that the battery is back alive, now I have to continue writing my review. 
Thanks for you inputs guys!