I assume you are referring to Net Express when you say "NEX"... FYI - the diminutive names we often use are "NX", "NetX" or "NE".

By default, Net Express reloads the last project that was used, so it's possible the project became corrupted to cause this error. To start Net Express without reloading the last project, you may open the Net Express 32-Bit Command Prompt and issue the command MFNETX /N