Recall where the headphones were last seen and carefully examine the area, including the floor, sofa, bed, table, etc.
Check to see if the headset has fallen into a gap or corner, and you can use a flashlight or phone flash to help find it. Try using the sounds your headphones make to locate it, such as playing music or making a beep.
If you can't find the headset, you can try using a Bluetooth tracker or app to locate it, provided the headset is Bluetooth enabled and paired.
Expand your search by checking other rooms or areas where headphones may be stored, such as backpacks, coat pockets, drawers, etc.