I have had this same problem: The main LimeWire window has disappeared and won't return, even though it's clear that the program is running in the background. I'm using LimeWire 2.3.4 with Mac OS 10.1.4. Here's what I've tried:

1. shutting down and restarting
2. making certain I have Mac's Java 1.3.1 Upadate 1 (I do)
3. making certain I have the "Shutdown Immediately" preference selected
4. uninstalling LimeWire and reinstalling
5. uninstalling and reinstalling in a different folder
6. deleting selected preferences files (any file that seemed as if could have been the cause)

None of these have worked at all. What's curious is that I can access the other windows (preferences, statistics), and it's clear that the program is there and running. Moreover, whenever I uninstall (and then manually trash the remaining LimeWire files) and then reinstall (even in a new folder), the program seems to be able to remember my previous preferences (even though I deleted the LW files, including the ".plist").

This is frustrating because I can't use LimeWire at all now. So, if anyone does have a fix, even a detailed one, I would love to hear it. Thanks.