According to Flightradar24, the last signal was received (thats ADB-out) at 13:32 about 30 km SW of Koblenz in Germany at 3600 ft. If this is really true, this sounds no good. The area there is in the hills and 3600 ft is certainly well below any safe altitude. And there is no airport near this spot that has a runway long enough for a B737.
The aircraft obviously descended to 10000 ft in the Schwarzwald aera (SW Germany) before and flew a circle near Zurich airport (Switzerland) before continuing with various headings towards Frankfurt and then further west.
Hope for the best.