Firstly, I want to say that I disabled the hybrid sleep, so I can choose between sleep and hibernate as I want. Previously the hibernate and sleep both works well and without problem, but now there is a problem with hibernation. Whenever I want to enter hibernation mode, the screen blackens for a few seconds, and login screen appears. It is as though I lock the screen or switch user. When I try to find the source of the problem, I saw that the hibernation file is smaller than my RAM size(RAM 1500MB but hiberfil.sys 1000MB), perhaps this is the problem?

There are several reasons can cause Hibernate issues. I suggest you refer the following article. How to troubleshoot hibernation and standby issues in Windows XP The article is also applied for Windows 7.Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.