It is the most frequent question in this forum!
There are already tutorials for that. The easiest fix is to reset the configuration. Follow the steps described here: resetting your OpenOffice user profile.
Note: reinstalling won't change anything if the profile is not deleted, which is not done automatically when removing the application.
A script has been developed, it may help you fix it automatically without resetting the whole profile: Spell checker "red wriggly lines" - beta test fix for AOO.
If no change, then follow this topic: [Troubleshooting] Spell check in OpenOffice.
Further information in this topic: [Solved] No dictionary present in OpenOffice.
If this sticky post with its links has helped you, don't hesitate to leave a post in this thread.