Ubuntu – apt-get: No such file or directory

i don't know why i suddenly can't do something with apt-get such as sudo apt-get update or sudo apt-get install blabla

this is output for uname -a :

Linux adelleodel-X450CC 3.16.0-30-generic #40~14.04.1-Ubuntu SMP Thu Jan 15 17:43:14 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

file apt-get :

apt-get: ERROR: cannot open `apt-get' (No such file or directory)

cat /etc/lsb-release:

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.3 LTS"

ls -l /usr/bin/apt-get :

ls: cannot access /usr/bin/apt-get: No such file or directory

dpkg -l apt :

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                          Version             Architecture        Description
+++-=============================-===================-===================-================================================================
rc  apt                           1.0.1ubuntu2.11     amd64               commandline package manager

echo $PATH :

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

i've searched before about this and some people said that i have to download something and dpkg it. how to do it then? can someone give the link and step by step how to do it?
Thanks a lot!!!!

—–editted—–
actually, i did sudo apt-get remove libusb-0.1-4:amd64 and some package autoremove with it i think..
i did that because i thought it wasn't belong with my program,.. because i have libusb-1.0-0:amd64, libusb-1.0-0-dev:amd64, and libusb-1.0-doc

i've tried wget http://security.ubuntu.com/ubuntu/pool/main/a/apt/apt_1.0.1ubuntu2.11_amd64.deb

the result is :

--2016-02-08 19:56:19--  http://security.ubuntu.com/ubuntu/pool/main/a/apt/apt_1.0.1ubuntu2.11_amd64.deb
Resolving security.ubuntu.com (security.ubuntu.com)... 91.189.91.13, 91.189.92.200, 91.189.92.201, ...
Connecting to security.ubuntu.com (security.ubuntu.com)|91.189.91.13|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 952468 (930K) [application/x-debian-package]
Saving to: ‘apt_1.0.1ubuntu2.11_amd64.deb’

100%[======================================>] 952.468      245KB/s   in 3,8s   

2016-02-08 19:56:29 (245 KB/s) - ‘apt_1.0.1ubuntu2.11_amd64.deb’ saved [952468/952468]

after that, sudo dpkg -i apt_apt_1.0.1ubuntu2.11_amd64.deb

the result is :

dpkg: error processing archive apt_apt_1.0.1ubuntu2.11_amd64.deb (--install):
 cannot access archive: No such file or directory
Errors were encountered while processing:
 apt_apt_1.0.1ubuntu2.11_amd64.deb
adelleodel@adelleodel-X450CC:~$ sudo dpkg -i apt_1.0.1ubuntu2.11_amd64.deb
(Reading database ... 230272 files and directories currently installed.)
Preparing to unpack apt_1.0.1ubuntu2.11_amd64.deb ...
Unpacking apt (1.0.1ubuntu2.11) over (0.8.10.3+squeeze1) ...
dpkg: dependency problems prevent configuration of apt:
 apt depends on libstdc++6 (>= 4.6); however:
  Version of libstdc++6 on system is 4.4.5-8.
 apt depends on gnupg; however:
  Package gnupg is not installed.

dpkg: error processing package apt (--install):
 dependency problems - leaving unconfigured
Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
Errors were encountered while processing:
 apt