This has happened to all of us.
You add a new object or make a change and then the element is gone. You know you didn’t delete it. But it’s gone. It’s not time to panic. It’s time to think… okay “It Makes Them Disappear”… what just happened? If it’s a wall, can you see it in 3D but not 2D? 2D but not 3D? If it’s a window, is it gone in 2D, but in the correct location in 3D? Is it not showing up in section? Find where you can see the object and then work backwards to find out what went wrong.
The ArchiCADWiki has a great article about locating an element that has gone missing: The list is broken down into 2D, 3D, and Layout.
Schedules are another place that items can go missing. This is often because your criteria for the schedule is excluding an object. Perhaps because of an ID or a wrong layer or some other element creation/classification error. Without fail, it’s usually an error that happens between the keyboard and the chair. Which honestly is great news. It means there’s very, very, very rarely some electrical gremlin inside ArchiCAD deleting your hard work. Embrace this knowledge. It is always hard to explain to a fellow ArchiCAD user that the problem they are dealing with is not an inherent bug, flaw, or inability of ArchiCAD, but just user error. People usually don’t like to hear that they are the problem. I for one see it in the reverse. It’s great if we are the problem. We can learn, improve, and discover other solutions. We can just stop adding that extra keystroke that messes up our operation.
Mastering ArchiCAD means more than wicked modeling chops and documentation skills.
Becoming an ArchiCAD expert is equally about problem solving and invention within the ArchiCAD and BIM environment. If errors, roadblocks, or mysteries appear, how do you resolve the situation? 
Just as impressive as being able to model this Queen Anne house (left) in ArchiCAD 15 are the detective skills required to solve this error (right). Want to know how to solve that missing attribute issue? Start at the ArchiCADWiki.
The true masters are not the ones with the answers, it’s the ones who can discover the solutions when things get rough. I can’t tell you the number of times someone has asked me a question and I didn’t know the answer. But by talking and searching, I am able to find a solution in real time. If you’ve ever e-mailed me, you’ve probably gotten an e-mail that starts with “well unfortunately that’s not possible because of… X, Y, or Z.” The middle of the e-mail then goes “so here’s a different route.” But by the time I finish writing, you’ll usually read something like “however, now that I think about it, you COULD do A, B, or C and get exactly what you want. It’s just a different angle than you were thinking.”
And that’s what I love about ArchiCAD. It’s not just about rotating the 3D model to get a different view, it’s about looking at the whole database of information in a different way to find the solution you’re searching for.
