Johan,
Firstly, check to see that the connections to the back of the HDD (Hard Drive) are okay. I'm going to assume that you have what we call a 'SATA' connection. Please see the below images, these should help you to better understand:
This is the standard SATA connection onto the back of your HDD and DVD Drive:
This is the other end of the connection, where is attaches to the Motherboard:
This is so that you know what each cable does:
Let us know if this connection is secure and we'll take it from there.
Firstly, check to see that the connections to the back of the HDD (Hard Drive) are okay. I'm going to assume that you have what we call a 'SATA' connection. Please see the below images, these should help you to better understand:
This is the standard SATA connection onto the back of your HDD and DVD Drive:

This is the other end of the connection, where is attaches to the Motherboard:

This is so that you know what each cable does:

Let us know if this connection is secure and we'll take it from there.