My glass fibre cables keep on disconnecting from machines they used to connect to, its not a just a visual thing because machines on the other end of the wire stop receiving power.
Known bug with ic2 in which the cable forgets that it is supposed to be connected to something due to a chunk loading issue... unfortunately it's not something we can fix. Whenever it happens, you'll just have to break and replace the piece of effected cable, and possible the machine block it's supposed to be connected to, in order to restore powerflow.