Attention: No video input

Hi,

From time to time when switch on my PC I got black monitor with a message “Attention: No video input”. It doesn’t happen all the time: sometimes everything works OK. I noticed that usually it happens after the PC was switched off for a half day at least. Then I switch it on and off number of times and it finally starts to work.

What is wrong? How can I fix it?

Thanks in advance.