Outdated Spotify app
Internet connection is slow
No Internet connection detected
firewall may be blocking
Visit Spotify Status's official Twitter account.
If the Spotify server is down you will get to know by some tweets by Spotify
Pay attention to the tweet timestamps to ensure that its not old
Turn off your router completely by taking out the power cable from the power switch
Now leave it for 2 minutes and then plug it back in
And check for your internet connection.