Something simple worth trying:
1) Quit Safari if it is running
2) From Finders "Go" menu, choose "Go to folder", type in ~/Library/Preferences, then click "Go" button.
3) Within the "Preferences" folder, sort by filename, then look for, and Trash, all the files that begin with "com.apple.Safari....". In particular, make sure to trash "com.apple.Safari.plist".
4) From Finders "Go" menu, choose "Go to folder", type in ~/Library/Caches, then click "Go".
5) Trash the "com.apple.safari" cache file
6) Try opening Safari again.
If that does not work, then I would recommend using another browser like Chrome (http://chrome.google.com) to download the latest version of Safari for your Mac OS version from (http://support.apple.com/downloads/#internet). Try installing it if it will alow it.
And lastly, if all else fails, backup your Mac, reboot, then download and install the latest "Combo Update" for your Mac OS version (http://support.apple.com/downloads/#combo). Installing or reinstalling a Combo update will almost certainly replace nearly many files associated with Safari and therefore would solve an issue where a critical file used by Safari is corrupted.