my computer monitor should turn on automatically when i turn on the power but sometimes it shuts down. why?

Answer:

It's not getting a signal from the video card. Perhaps the problem is within your system board, or your monitor.
there might be a loose power cable at the monitor / give up the monitor signal cable unplugged from the computer and you would see a check signal cable message on the monitor / leave the monitor surrounded by this state and see whether the monitor shuts down by itself / if yes , then surely you own problem with the monitor / else it might be a video card / past taking any descission it is recommended to use another available monitor and see whether the results are same / if the results are same on both the monitors then it is a video card / try reseating the video card / reseat system memory (RAM) / according to me it might be an memory or video card
Go into Start,control panel,enactment and maintenance ,power option's,set turn sour monitor,never / turn off tricky disks ,never / system standby , never.