Like many programs, NetBeans IDE automatically checks for updates. Like many programs built to be extensible, NetBeans updates components (or plugins) individually. So it came as no shock when I cranked up NetBeans 6.9 on my home PC this morning and it announced that there were 23 or so updates available. I told it to download and install them. Normally I get out of the way while NB does this, but I was in a bit of a hurry today, so I told it to download/install in the background while I worked on a small Java app ... which may have led to my downfall.
After it was done updating itself, NB called for me to restart it. I held off until I'd finished and saved some changes, then gave it the go-ahead. When it tried to restart, I was present with a very length list of error messages, all of which seemed to have something to do with either missing plugins or unsatisfied dependencies. I clicked past that, and NB opened up -- sort of. I didn't get the usual start page. I did get the file I'd been editing (a Java main class) and had left open, but the corresponding project did not open. So I tried to open it the usual way, but NB would not open it -- or any other Java project -- and did not seem to recognize them as projects at all.
Grump, grump: I bit the bullet, downloaded the Java SE version (all I need, and the smallest at about 54MB), uninstalled and reinstalled, but keeping my old settings (which, again, may turn out to have been shooting myself in the foot). The installer ran successfully and when I started it I got no error messages ... but also no projects. Once again, the file I'd been editing was open but I could not open any projects. This time, though, I noticed that when I highlighted a project directory there was a curious message in the open-project dialog that said something about a possibly uninstalled plugin (without specifying which). So I checked in Tools > Plugins > Installed and, lo and behold, 23 plugins (the correct number) were installed but most were disabled -- including all the Java development plugins.
So there's the dopey part: either the borked upgrade of the original copy of NB 6.9 changed a setting somewhere telling NB that none of the Java development stuff was active, or the 6.9.1 installer, designed for Java development, installed with the Java stuff inactive (which it's never done before). Dopey default? Or bad handling of upgrade errors? At least the fix is easy, once you recognize the problem.