NAND gate in proteus
The grey signals are "unknown", and they propagate through the gates, eventually being fed back to the input, so the unknown state persists.  This is something of a quirk with logic simulators, but also something of a warning.   It indicates that you may not be able to predict how the circuit powers up.  In real life, the state will randomly come up as 1 or 0, and the circuit proceeds from there.  But in the absolutely strict environment of the simulator, unknown is unknown.

You can get the circuit to "stabilize" by temporarily forcing those signals high or low (with more parts required).  You may also find some help by reading this question