Vertical lines on a monitor are caused by interference, improperly connected cables, hardware malfunction or out of date drivers. Older Cathode Ray Tube (CRT) monitors are more susceptible to interference than newer Liquid Crystal Display (LCD) monitors, making it worth troubleshooting the other possibilities first if you're using an LCD monitor.
Interference is caused by many common household items including cell phones, baby monitors or anything that produces a magnetic field, such as a ceiling fan. Moving these items away from the monitor will reduce the amount of interference. Faulty or half-connected cables can also produce unwanted vertical lines on a monitor. Disconnect the cables and visually confirm that the connectors are not bended or damaged. If feasible, connect the cable to another monitor to see if vertical lines persist. If the lines continue to appear, the cable is damaged and needs replacing. Most monitors have multiple video outputs, such as Digital Visual Interface (DVI) or High-Definition Multimedia Interface (HDMI). Use either connection (DVI or HDMI) to see if that particular port removes the vertical lines. If the monitor itself is malfunctioning, most retailers offer repair services or replacements if the monitor is still within the warranty period. The video drivers for your computer's graphics card may be outdated or corrupted. Check your computer's documentation to find out your video card manufacture and get the newest drivers from the manufacturer's website.