You need to be more descriptive. Usually ending explorer.exe - which can only be done in Windows - will only result in the graphical user interface disappearing, not in Windows being unable to boot. To restore explorer.exe while in Windows, do Ctrl-Alt-Del to get the Task Manager. Press the New Task button and type "explorer.exe" without the quotes. Press OK.
Please provide more details about your computer, the issue, and exactly what you did so we can help you. These links will show you what details to include in your answer:
http://www.elephantboycomputers.com/page2.html#Usenet (you can ignore the references to Usenet/newsgroups since you are posting on a web forum but the suggestions about how to write the post are applicable here too)
http://support.microsoft.com/default.aspx/kb/555375 - How to Ask a Question
MS-MVP - Elephant Boy Computers - Don't Panic!