Quark may need more memory: quit, check it, raise it if necessary. (I use 4x the suggested allocation, but start by doubling and see where that leads.) Quark may be tired: quit, restart, try again. Your image might be corrupt: "save as" with all extraneous info removed (alpha channels, unused paths, color profiles), or try saving as a tiff if possible. If I remember any other causes, I'll update.