You'll need to read your log to find out exactly why, but this is a common Jira startup problem when you haven't got a lot of resources allocated to it or you've got slow-loading plugins.
I've got http://blog.networkedcollaboration.com/2012/03/05/troubleshooting-jira-startup-errors/bookmarked to save me having to type the usual four things to try (they seem scattered in the Atlassian docs)
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