Minecraft is crashing after starting Multi- or Singleplayer

Since about a week I can't play Minecraft anymore.
First, I couldn't start it anymore because I deleted to .minecraft folder to check if it works but then there always were ''Loading versions'' and nothing happend.
I tried to turn off Firewall/Anti-Virus Program but it didnt help.
Then I asked my friend to send me his Minecraft files and now I can start it, but everytime when I try to start a singleplayer world it just crashes.
If I try to connect to multiplayer then it comes 'java.lang.nullPointerExecption :group'' and btw. all Servers are offline.
I already deleted and updated Java installed new Windows Updates etc.
Sorry for my bad english, I'm german.