Hallo Azhar,

if environment is not too dusty, I would try an optical system. If you put strong light at your cotton fiber thread, it should reflect or scatter some light back. You could use a laser pointer, like Joseph recommended, or just a very high efficiency LED with little radiation angle of about 20°...30°. Those parts produce an enormous intensity!!
Reflected and scattered light can be detected by simple photodiode, followed by amplifier and Schmitt-trigger. LED and photodiode should make such an angle to each other, as if cotton fiber thread would reflect the light. Then efficiency is highest. In example below, they make an angle of 90°. Hereby LED axis makes an angle of 45° with cotton fiber thread:



Of course, whole arrangement must be well shielded against ambient light. Also, photodiode must be very well shielded against LED! Relevant intensity may only reach photodiode, when cotton fiber thread is existent.

In industrial environment dust and dirt forbid optical systems very often. So, if this is the case in your application, I would proceed with the idea of Terry.

Kai