I got bit by an old bug recently. To be fair, it's not a bug, it really is a feature, but that didn't make the bite feel any better and "bit by a feature" is a confusing metaphor.
If you accidentally turn on "Account Security" in the Company->Company setup window all kinds of mayhem ensues.
See, account security is designed to work with the Organizational Structure feature to limit access to certain accounts. The idea behind this is that some organizations have a very complex account structure and it makes picking the right account easier if you limit a department to only be able to see their own accounts. But if you haven't setup Organizational Structures (a pretty common occurrence), turning on Account Security will wipe out everyone's ability to see and interact with the chart of accounts.
Oops. It sounds like a good thing to turn on. I had this happen at a client recently and was stumped as to what had changed. I remember this scenario but it had been a long time. The client is new to GP and so they are not sophisticated enough to have found and turned off security to the Chart tables in GP10. Heck, I'm not sure if I could find that without some digging in Version 10, I certainly couldn't stumble across that accidentally.
A quick trip to Partnersource reminded me of this issue and a checkbox later, we were good to go. We didn't have audit trails on yet so I'm not sure (nor do I really care) who turned it on but we'll be adding that to the audit trail list.