Fixing over discharged battery

Hi, i had my drone crashed for 1 year ago, and since it it has been laying waiting to be fixed. I got it fixed yesterday, but i didnt preserve my battery and it is dead. It had no lights whatsoever. Since it didnt took any charge, i decided to open it and charge it cell by cell, i managed to charge them up to 3.53-3.55V each. Now i got lights. But i still cant charge it, when i connect charger it just blinks faster for few seconds and after that blinks slowly. Just like if you turned on the battery. What could be the case?