Although sealed Lead-Acid batteries do suffer from sulfating another common problem is that since they are operated under starved electrolyte conditions the electrolyte can end up as pure water that does not conduct. (all the sulphate ions are in chemical combination with the lead in the electrodes).
To rectify this situation you may need to charge it with a high-voltage (20-30V) current limited supply (put 1k in series) and leave it for a few weeks.
If you are lucky there will be enough current to reconvert the water to sulphuric acid and restore the battery to operating condition. After that you can charge normally.
Although everyone talks about "sulfation" as a problem, the normal operation of the battery converts the lead/lead dioxide in the plates to lead sulphate as it reacts with the electrolyte to create electricity, it is not in itself a problem condition. What does happen if the lead sulphate is allowed to sit for a long period it can convert into a form that is not easy to transform into lead/lead dioxide during the charging process.