Pc doesn't detect my S7 I have a Samsung S7. When I connect the phone with my Pc to transfer work datas ( I use a USB cable), it doesn't detect the phone. 
How do I fix this problem?
You need to install appropriate usb drivers for the PC to detect your device.

It may be that  your PC is missing the required USB drivers or they have become corrupted, thus unable to recognise the device

If running Windows install the Samsung USB driver (includes MTP drivers as well).

Once installed reboot your PC and reconnect your Android device, it should be recognised.

Alternatively for Linux the guide for installing the drivers is found here: How to setup ADB/USB drivers for Ubuntu

Since you didn't mention your OS I have to assume Windows or Linux (sorry couldn't find MacOS instructions)