Since you are asking this here and did not say you tried to reconnect the system and controller via the sync buttons on each, I will advise first to do this. If this does not work, restart your system. Many console errors such as controllers failing to reconnect and unresponsive to a connected controller (i.e. system freezing) is easily corrected by restarting. If a restart does not fix the issue, change your batteries. Dying batteries can cause a lot of problems with connecting controllers and starting the system with the controller. I recommend actually investing in the rechargeable batteries because then you can charge and play without wasting money on AA batteries. If you use your XboX like my family does, it is a worth while investment to buy two battery packs and the stand. You can also buy the battery pack with USB cord if you only want to have a single battery.