Its because your resolution is set too high for your monitor to cope with, although this would not explain why internet being disconnected would cause it.
Solution would be to boot to safe mode (f8 key) Right click on the desktop and click Properties, and then on the Settings tab.
Now click and drag the Screen Resolution slider to the far left ... you want the smallest settings that your screen will support. Typically that'll be 640x480, or 800x600.
Also change the Color Quality dropdown to 256 colors, or whatever is lowest. (You probably don't need to go as low as 16 colors, if that's presented as an option.)
Now, click on the Advanced button, and then the Monitor tab of the resulting dialog box. Change the Screen refresh rate to 60 Hertz.
After that experiment to find what the highest setting your pc can cope with