How to keep led in "ON" state using 555 IC?
A 555 isn't really required for a simple task like this.  You can make the LED latch with another transistor and a few extra components:



When the voltage at the junction of the photoresistor and R4 rises high enough, Q1 turns on, turning on the LED.  Q1 pulling low also turns on Q2, which charges C1 and latches Q1 in its ON state.  Discharging C1 with the switch turns the LED off, and assuming it is sufficiently light (the resistance of the photoresistor has decreased), Q1 and the LED will remain off.