There has recently been a security update for Raspberry Pi. Here is an article from https://www.raspberrypi.org/blog/a-security-update-for-raspbian-pixel/
From the foundation's article:
What has changed?
First, from now on SSH will be disabled by default on our images. SSH
(Secure SHell) is a networking protocol which allows you to remotely
log into a Linux computer and control it from a remote command line.
As mentioned above, many Pi owners use it to install a Pi headless
(without screen or keyboard) and control it from another PC.
In the past, SSH was enabled by default, so people using their Pi
headless could easily update their SD card to a new image. Switching
SSH on or off has always required the use of raspi-config or the
Raspberry Pi Configuration application, but to access those, you need
a screen and keyboard connected to the Pi itself, which is not the
case in headless applications. So we’ve provided a simple mechanism
for enabling SSH before an image is booted.
The boot partition on a Pi should be accessible from any machine with
an SD card reader, on Windows, Mac, or Linux. If you want to enable
SSH, all you need to do is to put a file called ssh in the /boot/
directory. The contents of the file don’t matter: it can contain any
text you like, or even nothing at all. When the Pi boots, it looks for
this file; if it finds it, it enables SSH and then deletes the file.
SSH can still be turned on or off from the Raspberry Pi Configuration
application or raspi-config; this is simply an additional way to turn
it on if you can’t easily run either of those applications.
The risk with an open SSH port is that someone can access it and log
in; to do this, they need a user account and a password. Out of the
box, all Raspbian installs have the default user account ‘pi’ with the
password ‘raspberry’. If you’re enabling SSH, you should really change
the password for the ‘pi’ user to prevent a hacker using the defaults.
To encourage this, we’ve added warnings to the boot process. If SSH is
enabled, and the password for the ‘pi’ user is still ‘raspberry’,
you’ll see a warning message whenever you boot the Pi, whether to the
desktop or the command line. We’re not enforcing password changes, but
you’ll be warned whenever you boot if your Pi is potentially at risk.
Our hope is that these (relatively minor) changes will not cause too
much inconvenience, but they will make it much harder for hackers to
attack the Pi.
What this basically means is that there has recently been lots of hacking of devices using remote connection like SSH. Due to this, SSH is by default turned disabled. To fix this all you have to do is create a file called ssh in the
/boot/ directory. This file called ssh can contain any text or nothing if you want. Then, using the
sudo raspi-config command on the terminal you can enable ssh again.
Also, it recommends you change your password if it was something simple like password to something more difficult to crack using the