Ok so i'm at the 7th gym and I want to teach my steelix iron tail but the tm DISAPPERED!!! I check all of my Pokemon to make sure that I didn't use any, I didn't. This is in heart gold btw.
Your Iron Tail TM probably didn't just disappear, (Unless there was some sort of glitch.) you either tossed or sold it. You won't be able to get another Iron Tail TM however, Steelix learns it at level 41.
Hope this helps!