You can use "in bed" or "on her bed", not "on bed".
If she is lying on top of the covers, she is lying on her bed.
If she has the covers pulled over her, she is "in bed".
You can use "in bed" or "on her bed", not "on bed".
If she is lying on top of the covers, she is lying on her bed.
If she has the covers pulled over her, she is "in bed".