My computer turns on but then immediately shuts down.

Question

It seems my computer turned itself off in the middle of the night. This morning, when I tried turning it on, it wouldn't leave the white initial screen with the apple logo and the waiting sign so I manually shut it down. I waited a couple of minutes before doing so. Then I turned it back on again and the same thing happened. The third time I turned it on, the white screen appeared and then it shut down on its own! Next time I did the same thing and it took less time before shutting down on its own.
My computer is connected to the energy source. It's not connected directly to the wall, there is a no-break with a very weak battery and that's connected to the power source.
My concern is that there was very little memory left on my computer and I was downloading more movies.
It the problem coming from my computer or is it the energy source?

Answer 1

Try connecting the plug directly to an outlet. That should determine whether it is the energy source or not.
If the same, try booting into the Safe Mode using your normal account.  Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.
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