You might have had an error message pop up regarding a “cyclic redundancy error”. Known as a CRC, it indicates that the data on your drive cannot be read due to a malfunction. This is usually a sign of a failing hard drive.
The one mistake many users make is to panic and copy their data to an external hard drive. This is something you should not do, despite it seeming like the obvious thing to do. Doing this puts strain on an already failing hard drive, and can be catastrophic.
Instead, you should take a clone or image – this way the heads will work slowly, to produce a RAW picture of your data which can easily be rebuilt. Stella Phoenix and ddrescue are two programs that are free to download and can be used to image a hard drive easily.