If you have created a forum that does not have a topic within it, students will not be able to see and post to the forum.
If you have created a topic for your forum, you may have accidentally put a time restriction for the students to be able to view the topic. Edit that particular topic and scroll down to the Availability section. Make sure the topic is not hidden and that it has the correct start and end dates.
The forum or topic may also have viewing restrictions based on groups. Edit the particular topic or forum and go to the Restrictions tab. Make sure that the appropriate Group Restrictions are applied. In addition, check to make sure that the user or users who are unable to see the topic are properly enrolled in Groups, if group restrictions are present. See Groups Documentation for more details.