Hard disk not found, but works in external disk reader

So out of nowhere, my Dell Inspiron 15z stops booting from my hard disk. I do my research and find the most common suggestion is to take the whole laptop apart and reseat the hard disk. I do this and receive the same error. My lady is freaking out because she had a semester's worth of work on the computer, and she didn't back it up. Lesson learned there. In the meantime, we bought an external hard disk reader and got her stuff off the hard disk. I tried reseating the hard drive again today, put the computer back together and am still recieving the same error message. So if the hard disk works in the reader, but the laptop isn't finding it, any idea what my problem might be?

Thanks.