[Magic Touch] The image have suddenly disappeared (the screen went black). What should I do?

You need to restart iPhone/iPod. Alternatively you can press Image->Reload, however without iPhone restart the black screen is likely to appear again. Another option is to select smaller canvas size (Tools->Misc->Canvas select 160x120) and re-open image via Image->Open. Still, quick iPhone restart will be the best possible solution.?Also, remember to save your image regularly (it's quick: Image->Save), so that next time you can safely re-open it.