Hmm, interesting. As I don't use Keil C52 (because I prefer writing everything in pure assembly), I can't tell you the exact cause of the problem. As you've probably learn't from this exercise, it is always I good idea to make sure you save the last working copy of your code (preferably with a different filename to the code you are changing). There is nothing more annoying than when your code works, then you make a small change and suddenly nothing works at all. It happens to all of us! Saving work can save a lot of bother!

If you are just setting about learning to program the 8052, I would recomend getting to grips with a bit of assembly language other than starting with C. It will give you a much better understanding of what is going on inside the microcontroller, and this knowledge will come in very hand when your C compiler starts doing strange things, like the problem you are experiencing at the moment.

Matt.