You don't need to open battery case to measure individual cell voltages. That's what balance connectors are for. Anyway, it's now pretty much late to anything. If you slowly discharged them by leaving controller on and they haven't puffed you can try to trickle charge them, but they are not to be trusted anymore. If you give it a go at charging do that outside and at fire-safe place, because charing overdischarged LiPo is almost certainly going to end in a huge fireball.
If you want to charge them use charge only mode on your charger (without connecting balance connector), and select 2S, then 3S, and that way you'll slowly bring up the voltage. Select low charging current, start with 0.1A.
RWD 2807 9Continent rolling on Kenda Flame 26x2.125
12S3P out of Turnigy 4S Hardcase 20C (600Wh)
35A 9FET Infineon Controller (Regen, Cruise control, Speed limit)
Top Speed 27mph (31mph@120%)