I can not play online on minecraft

One day i was installing Mods and it deleted something on the .minecraft folder and ever since i can not play online, because i log in and it says, "Login in failed" and then you had a option saying play offline and i have tried to delete .minecraft so i can install minecraft again but it wont let me and it says, that the folder you are trying to delete is open and i checked and its not!

Please Help me!