I have a 42inch lcd polariod tv and the screen comes up blue no picture or sound

It could be the blanking circuit displaying a blue screen if no or a weak signal is connected to the TV. If you have a DVD or VCR try to see if you get picture and sound when ether of those devices are connected to the auxiliary input. If yes your signal input is weak, or you are getting no signal at all. If you go into the TV do you see text? If yes make sure you’re on the correct auxiliary input. If no text is visible the problem is internal and the LCD driver circuit needs to be repaired. If you see text and you are on the correct auxiliary input, but you get no picture and sound then the problem is internal and the front end tuner needs to be repaired. If you see video and hear the sound make sure your channel is correctly selected and set to a working channel. If you have a cable box remove the input to the box and connect it into the coaxial input on the TV and see if you have a picture and sound. If you do the cable box is defective. If you do not see any channels the cable may have lost its signal, and you should connect the cable to another device to check the signal quality. If you have a satellite service provider, and you use a known working device to check the signal and the device works then the TV needs to be serviced.