|Is your computer even working--does anything ever show on the screen other than those messages?|
The 'check video cable' message is coming from the monitor. The 'going to sleep' message may also be a monitor message.
If your keyboard has the power management keys:
you might see if you can use the 'wake' key to get the computer going. But I suspect your computer isn't working and the messages are generated because the computer isn't sending a signal to the monitor.