how to avoid crystallization of powdered sugar
First, if you powdered it yourself, you should have added cornstarch, else it goes stone-hard quickly. 

Second, you have to keep it away from moisture. The bottle would have to be closed tightly, and you have to use a perfectly dry bottle, not a just-washed one. If you live in tropical region or somewhere else with really high moisture, a bottle may not be a good container, try something wide mouthed from which you can break out the sugar if needed. 

For the existing bottle, you might not need to discard it. Try soaking it in a larger water container overnight. With luck, it will dissolve the sugar enough to get it (partly?) out. The resulting sweet liquid can be boiled down to make simple syrup or caramel. You might need to boil it for a long time though, depending on the amount of water used.