Can you help me out?

Do you have a modded version of minecraft? How did you get this bug?

My Minecraft is not modded and I have no idea how this happened.

Can you start minecraft?

Everything works fine until I join a server or play single player.

I tried deleting and downloading minecraft again but it didn't help.

What should I do?

I'm sorry I can't help you, but this forum is for bugs in minecraft, not support. For technical support please use the Mojang Support Center.