Assuming you are on QuickBooks Desktop, this could be a problem with your file. It's happened to us a few times and there's usually a quick fix. First, make sure you have a good backup. Then, open your Chart of Accounts and at the bottom there are a few menus, select [Account] -> [Resort List].
If this doesn't solve the problem, you may want to rebuild your file.
You can do a verify first by selecting [File]->[Utilities]->[Verify Data]. This goes through your data file and tries to locate any inconsistencies. To rebuild the data and correct any problems, select [File]->[Utilities]->[Rebuild Data].
