My mode was Sandbox. Mechjeb was the first and only mod I inserted. But then, it hid all the parts for building a Spacecraft. Fix it! I am so loving the mod but the mod started to destroy the codes. I was using the latest KSP Version (1.0.4).

I can't do anything without a log.

sarbian, what do you mean?

I don't have that at all. I can't fix it if I can't duplicate it or at least read a log where it happened.
So duplicate the bug and send me the KSP log so I can fix the problem.

I am now closing this because I found the solution. I uninstalled and installed both KSP and the mod.