I just performed apple's software update last night. This morning, when going to open a png file I created last year, I was surprised the application crash. Next time I opened the file, it allowed me to select a layer, then it crashed. Luckily, many newer png files I've checked seem to be working fine. Also, png files without layers or text seem to open fine too.
I'm using Fireworks MX but have recently upgraded DreamWeaver to CS4.
Make sure the PNG files are in the same location they were when you last opened them, and that you haven't removed any fonts you used in creating them. A corrupted font could also be the problem.