Assuming it was working before and some changes were made in the bios. - Pop open the case and clear the CMOS by taking out the coin sized battery on the motherboard and wait a few seconds then put it back in. Test to see if your pc works. If it worked, then it was a setting you've changed in the BIOS settings. Assuming you are using an external graphics card and have an Integrated GPU. - Take out the external card and plug your monitor to the IGPU. If it works then your may be GPU is fried. Assuming your PC is perfectly fine. - Take your pc and plug it into your TV or any other monitor to see if it works. If it works then try connecting something else (such as a gaming console or another computer) to your monitor to see if it works. If it doesn't work with other devices, then your monitor is fried. Assuming everything is good. - Select the correct input on your monitor. Replace all cables and try again. 
When i turn on my pc it says check signal cable then just enter lock screen. Im not avaible to enter bios or enywhere else its just shows check signal cable. The biggest problem is i cant enter bios and when i boot from DVD or CD or USB its still show check signal cable. I use dvi adapeter with vga and i have LG monitor. This worked before.
DELL and Windows boot logo dont show up at startup!