Did you install any new assets recently? (Obviously shouldnt screw it up, but just checking) If you force quitted Trainz recently after it froze or crashed while having Itrainz opened, that might explain it.
Also, what colour is the Itrainz button? I've found sometimes that if it is grey (Meaning you are offline), then it doesnt like to open. Close Trainz, fix your internet connection the TRAINZ servers and then load up Trainz again.