The former Rennes star, who only joined Dortmund last summer, was absent on Thursday with the club struggling to find his whereabouts.
Bosz tried to contact the France international by phone but the Barcelona target has yet to explain his actions.
Reports in Germany claim Dortmund have rejected a bid from the Spanish giants and there are rumours claiming he is on his way to Barcelona.
Barcelona have identified Dembele as a replacement for Neymar but Bosz is demanding talks with Dembele.
He told reporters: "Dembele wasn't at training today. I have no idea why he wasn't there. We tried to reach him but couldn't.
"First I have to talk to Dembele, then he has to explain to me why he wasn't here today. I hope nothing serious happened."