Okay, my initial suspicion is you lost your queen somewhere along the line. If you have no brood. That may be what happened. If the levels of Terramycian are to high that may also be an issue. I don't know what or how much they use on the dandelions but I wouldn't rule it out.
As for now, do not put anything from the dead hive into the good hive.
Do any of your other hives show any problems?
How close together are your hives kept?