1. Monitor problem(as you said monitor iam assuming its a desktop)
check monitor; Remove VGA cable(blue in color) from the back of CPU/Chassis/box coming from the monitor. Power on the monitor and not the computer/cpu/chassis . If monitor does not show any lines and says that cable not connected or shows some menu as no input then monitor is probably fine. Reattach VGA cable. Power on the computer and press or tap F2/Del key to get in to BIOS settings. Once you have entered Bios check if you see any lines or colors if not monitor is fine.
2. video driver/card problem
Restart computer press/tap F8 key till you reach a screen that says safe mode, safe mode with networking etc on a black screen. Select Safe mode and hit enter. Dont worry if you see everthing is in enlarged form, it just means that your computer is running with your mother board integrated video and not the add on video card itself.
do you see lines now. No , your motherboard is fine, could be a problem with either video card drivers or video card
Reinstall video card driver. Control panel> system> select hardware tab>device manager> select your video card (display adapter) right click on it and install video card rivers.
Make sure if you do not have a video card, you have installed Video drivers for your motherboard from the CD that you got when you bought your computer. Dont tell me that you dint get it , everybody gets its and then they forget about it,
Remove video card Manually from CPU chassis and check if you still receive lines. if no, video card could have gone bad
3. motherboard(integrated video) , very rare
If you see lines in BIOS or F8 screen then probably motherboard has gone bad.
4. no high voltage appliance near monitor(vacuum cleaner,hair blower etc)
You can also check for monitor problems if you have an extra monitor or laptop, just connect VGA/DVI cable to laptop or extra monitor and check.
press Fn+f8 for laptop display
reply if any prob.