being in a trance or a state of mental confusion, such as after a shock or extreme fatigue

突如其来的噩耗让她神思恍惚，仿佛丢了魂一般。

being abstracted or absent-minded, often unable to concentrate due to preoccupation

他由于心事重重，整天神思恍惚，连工作都出了不少错。