Hi Rose Marie,
Unfortunately there are many reasons why the hard drive missing message could be coming up, so we can't just tell you, "do this" and it'll be fixed. Dell has put together a whole page dedicated to troubleshooting that problem on their laptops, so that's where I would suggest you start.
[link|https://www.dell.com/support/kbdoc/en-us/000132300/how-to-troubleshoot-hard-drive-or-solid-state-drive-issues-on-a-dell-computer|How to Troubleshoot Hard Drive or Solid-State Drive Issues on a Dell Computer - Dell US|new_window=true]
Go through those steps then come back and let us know what you found. If you haven't managed to fix it with those instructions, we may be able to help further, and if you did get it working, let us know what the problem was and how you fixed it; that way someone else who has that same problem in the future can benefit from your experience!