My Adsense ads stopped showing up on some of his pages, and then stopped showing up altogether. Why?

This problem is isolated to your colleague's computer and you are able to see ads on your computer. I have several suggestions which might clear up this problem.

First, have clear your browser cache and delete the browser cookies on both IE and Firefox. This sometimes solves the problem. Next, make sure that JavaScript is enabled on the browsers.

If the problem persists, I suggest taking the following measures:

1. Scan your computer for viruses, making sure that any anti-virus programs on his computer contain the most recent updates.
2. Download and run an anti-spyware program to remove any adware and/or spyware from his computer.