Hard to say...probably in shock. When you found the tank cold, the first reaction is to raise the temp back to normal quickly. That increases the stress. The best thing to do would be to increase the temp over a long period......Increasing the Oxygen in the tank with an air pump can't hurt.
Sounds like you need better temperature control. Thermostats on heaters are notoriously bad, I would invest in a better heat controller. Then set the controller on the heater as a safety it something sticks on.