every once in a while when i use my external HD, a similar situation happens.
If your USB cable has two tails, try plugging them both in (to the laptop), the extra power might jump start it.
Another solution is to plug it in, set the PSP to usb mode, then put the macbook to sleep by closing the lid, wait a few seconds until it completely sleeps, then wake it up again.
If you can't figure out how to get it to show up on your Macintosh, try looking at Sony's PlayStation Portable Support Page [link] or you can send an email to Customer Services [link]