Why does the computer go black after log in?

It's hard to say. It could involve a resolution or monitor output setting. Most laptops, especially new ones give the option of using a television or other unattached monitor to use in addition to the attached monitor. However, there is usually an option that makes the attached monitor blank while projecting the desktop onto the alternate monitor. I am only throwing this out there because it is an unobvious possibility that can be easily resolved from safe mode by right clicking the desktop and fiddling with monitor settings. On my laptop, there is a variety of settings that involve monitor resolution and monitor additions which I have found to be both highly customizable and an extraordinary pain in the @R5E.

Or...It could be worse.

Edit: ASAP. Back up your harddrive, just in case.

If the OS is messed up, you may have no choice but to reinstall it. Luckily, you should be able to back up your hard drive before you do this and won't lose anything.

Once you bypass the login screen, the windows running system reverts your access to the desktop page. It could be a running error. Try rebooting the laptop into an earlier time that it was running well. What you do is you will turn on the computer and tap, either F10 or F11 until the restoral program boots. Then select restore to an earlier time. A few minutes and your computer will be running as it was before.

check setting for power. may be setting is like to turn off monitor after some minutes.