You could try reset and update in terminal.

Thanks for the answer!
I'm sorry, I'm a newbie, so can you tell me what t type exactly? I tried "sudo reset" but I didn't know what to type after. Then I tried apt-get update but it gave me this:

Ign:1 http://ppa.launchpad.net/no1wantdthisname/ppa/ubuntu trusty InRelease
Ign:2 http://ftp.belnet.be/deepin unstable InRelease                  
Ign:3 http://dl.google.com/linux/earth/deb stable InRelease            
Atteint:4 http://ppa.launchpad.net/no1wantdthisname/ppa/ubuntu trusty Release
Ign:5 http://ftp.belnet.be/deepin unstable Release                     
Atteint:6 http://dl.google.com/linux/earth/deb stable Release         
Réception de:7 http://ppa.launchpad.net/no1wantdthisname/ppa/ubuntu trusty Release.gpg [316 B]
Ign:8 http://ftp.belnet.be/deepin unstable/main amd64 Packages         
Atteint:9 http://repository.spotify.com stable InRelease               
Ign:10 http://ftp.belnet.be/deepin unstable/main i386 Packages     
Ign:12 http://ftp.belnet.be/deepin unstable/main all Packages   
Ign:13 http://ftp.belnet.be/deepin unstable/contrib amd64 Packages
Ign:14 http://ftp.belnet.be/deepin unstable/contrib i386 Packages
Réception de:15 https://deb.opera.com/opera-developer stable InRelease [2 592 B]
Ign:16 http://ftp.belnet.be/deepin unstable/contrib all Packages      
Ign:17 http://ftp.belnet.be/deepin unstable/non-free amd64 Packages
Ign:18 http://ftp.belnet.be/deepin unstable/non-free i386 Packages
Ign:19 http://ftp.belnet.be/deepin unstable/non-free all Packages
Ign:8 http://ftp.belnet.be/deepin unstable/main amd64 Packages   
Ign:10 http://ftp.belnet.be/deepin unstable/main i386 Packages   
Ign:12 http://ftp.belnet.be/deepin unstable/main all Packages
Ign:13 http://ftp.belnet.be/deepin unstable/contrib amd64 Packages
Ign:14 http://ftp.belnet.be/deepin unstable/contrib i386 Packages
Réception de:20 https://deb.opera.com/opera-developer stable/non-free amd64 Packages [1 816 B]
Ign:16 http://ftp.belnet.be/deepin unstable/contrib all Packages  
Ign:17 http://ftp.belnet.be/deepin unstable/non-free amd64 Packages
Réception de:21 https://deb.opera.com/opera-developer stable/non-free i386 Packages [1 814 B]
Ign:18 http://ftp.belnet.be/deepin unstable/non-free i386 Packages     
Ign:19 http://ftp.belnet.be/deepin unstable/non-free all Packages
Ign:8 http://ftp.belnet.be/deepin unstable/main amd64 Packages
Ign:10 http://ftp.belnet.be/deepin unstable/main i386 Packages
Ign:12 http://ftp.belnet.be/deepin unstable/main all Packages
Ign:13 http://ftp.belnet.be/deepin unstable/contrib amd64 Packages
Ign:14 http://ftp.belnet.be/deepin unstable/contrib i386 Packages
Ign:16 http://ftp.belnet.be/deepin unstable/contrib all Packages
Ign:17 http://ftp.belnet.be/deepin unstable/non-free amd64 Packages
Ign:18 http://ftp.belnet.be/deepin unstable/non-free i386 Packages
Ign:19 http://ftp.belnet.be/deepin unstable/non-free all Packages
Ign:8 http://ftp.belnet.be/deepin unstable/main amd64 Packages
Ign:10 http://ftp.belnet.be/deepin unstable/main i386 Packages
Ign:12 http://ftp.belnet.be/deepin unstable/main all Packages
Ign:13 http://ftp.belnet.be/deepin unstable/contrib amd64 Packages
Ign:14 http://ftp.belnet.be/deepin unstable/contrib i386 Packages
Ign:16 http://ftp.belnet.be/deepin unstable/contrib all Packages
Ign:17 http://ftp.belnet.be/deepin unstable/non-free amd64 Packages
Ign:18 http://ftp.belnet.be/deepin unstable/non-free i386 Packages
Ign:19 http://ftp.belnet.be/deepin unstable/non-free all Packages
Err:8 http://ftp.belnet.be/deepin unstable/main amd64 Packages
  404  Not Found [IP : 193.190.67.98 80]
Err:10 http://ftp.belnet.be/deepin unstable/main i386 Packages
  404  Not Found [IP : 193.190.67.98 80]
Ign:12 http://ftp.belnet.be/deepin unstable/main all Packages
Err:13 http://ftp.belnet.be/deepin unstable/contrib amd64 Packages
  404  Not Found [IP : 193.190.67.98 80]
Err:14 http://ftp.belnet.be/deepin unstable/contrib i386 Packages
  404  Not Found [IP : 193.190.67.98 80]
Ign:16 http://ftp.belnet.be/deepin unstable/contrib all Packages
Err:17 http://ftp.belnet.be/deepin unstable/non-free amd64 Packages
  404  Not Found [IP : 193.190.67.98 80]
Err:18 http://ftp.belnet.be/deepin unstable/non-free i386 Packages
  404  Not Found [IP : 193.190.67.98 80]
Ign:19 http://ftp.belnet.be/deepin unstable/non-free all Packages
6 538 o réceptionnés en 2s (3 051 o/s)
Lecture des listes de paquets... Fait
W: The repository 'http://ftp.belnet.be/deepin unstable Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: Failed to fetch http://ftp.belnet.be/deepin/dist ... nary-amd64/Packages  404  Not Found [IP : 193.190.67.98 80]
W: Failed to fetch http://ftp.belnet.be/deepin/dist ... inary-i386/Packages  404  Not Found [IP : 193.190.67.98 80]
W: Failed to fetch http://ftp.belnet.be/deepin/dist ... nary-amd64/Packages  404  Not Found [IP : 193.190.67.98 80]
W: Failed to fetch http://ftp.belnet.be/deepin/dist ... inary-i386/Packages  404  Not Found [IP : 193.190.67.98 80]
W: Failed to fetch http://ftp.belnet.be/deepin/dist ... nary-amd64/Packages  404  Not Found [IP : 193.190.67.98 80]
W: Failed to fetch http://ftp.belnet.be/deepin/dist ... inary-i386/Packages  404  Not Found [IP : 193.190.67.98 80]
E: Le téléchargement de quelques fichiers d'index a échoué, ils ont été ignorés, ou les anciens ont été utilisés à la place.

What does this mean? Why so many errors? Do you know how to fix this?

Thanks again!