So you have an assignment that students can't see. There are many different issues that can block students from accessing assignments. Below are some examples of what they may see and how it can be fixed.
Students see "Access Denied"
If you send the students the link to the assignment, and they're saying they're seeing a lock and/or "Access Denied," that means there is something unpublished. It may be the assignment itself that's unpublished, either in the Modules tab or the Assignments tab. It may also be that the assignment is in an unpublished module. See below for an example: It doesn't matter if those assignments are published everywhere else. The module they are in is unpublished, so students will not be able to see them, even when you give them the link.
Click the 
to publish that module, or move the assignment in question to a published module, and they will be able to access it.
Students see "Error while processing or retrieving the document"
If they see this, the first thing to check and change is the assignment's uploaded file. Odd file types can cause issues like this. The best course of action is to find the file, either in the Files tab of the course or wherever you uploaded it from, switch it to a PDF file, and reupload it.
Students see "Error 401" or "Error 404"
These errors can be caused by many things. In general, though, it's a good idea to have students do the following whenever they see errors or any other oddities.
- Try a different web browser
- Clear the browser's cache and cookies
- Check that the browser is up to date
- Check that the device is up to date
- Turn the device off and on. Not a restart, but fully powering it off.
Other possibilities
It could be that your assignment isn't fully published.
It could be that your assignment resides in the Assignments Tab but is not in the Module Tab
Of course, there are a multitude of tech issues, and you or your students may see something that wasn't described here. Whenever something weird happens, the best bet is always to follow what was described in the previous section, particularly trying a different browser and restarting the device. Beyond that, feel free to submit a Solutions ticket, describing the issue with as much detail as possible.