Storing it should matter as long as you gave it some time to settle before powering it up and didn't have anything resting on it. Plasma TVs contain neon or xenon gases that tend to pool up when stored in a non standard way. Those screens are very fragile and even the slightest pressure over time can damage them. You could try a Factory reset and recalibration but if that doesn't work then maybe it's time to retire it, generally they are not worth repairing considering the power usage and limited factors that go along with Plasma.