When I start my pc, a message appears “operating system not found”

At First start your PC and by pressing Dell or F2 enter into system setup and check your hard drive detection. In advance system option check for Primary or secondary drive. If your data cable and power supply connector is connected accurately with hard, then here you will see the capacity of hard drive. In this case you have lost your operating system and so install again windows. 2^nd: If you didn’t find any hard drive detection in setup, it mean that cables are not connected with hard drive. Also there could be any physical problem in your hard drive.