But before posting, try to update the graphics card driver - this was the most common source of problems in previous SE versions, so may apply to the current one.
Please post a bug report with a more or less detailed explanation, screenshot, and the log file. It is located at
<Your steam directory>\steamapps\common\SpaceEngine\system\se.log
Suggestions by users:
Maybe some info for others (and the team): a lot of people with high-end systems and/or more then enough RAM, have disabled the Page File within Windows. Turns out that (at least with NVidia drivers) that causes crashes and/or total system freeze with SE. Since I did enable Page File again (just let Windows manage it), no more crashes and freezes
SpaceEngine don't start Obsolete graphics card