There may be following issues :
1- There may be plugin conflict. To check this disable all the plugins and check if it works. If it works then enable the plugins one by one again and get the plugin which is making the issue.
2- Check if there is any issue in the active theme. To check this switch to the default theme. Check if the issue is gone. If the issue is gone then check for the issue in the old active theme.
3- Check for the Roles and Permissions.
4- Clear the cache.