Failed to start A high performance web server and a reverse proxy server

My nginx application is Working fine but when i try to restart service it shows the Following error. Here is my output

root@gokul:~# systemctl status nginx
● nginx.service - A high performance web server and a reverse proxy server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2018-12-23 08:50:34 IST; 3s ago
     Docs: man:nginx(8)
  Process: 4638 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=1/FAILURE)
  Process: 4637 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
 Main PID: 795 (code=exited, status=0/SUCCESS)

Dec 23 08:50:31 gokul systemd[1]: Starting A high performance web server and a reverse proxy server...
Dec 23 08:50:31 gokul nginx[4638]: nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Dec 23 08:50:32 gokul nginx[4638]: nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Dec 23 08:50:32 gokul nginx[4638]: nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Dec 23 08:50:33 gokul nginx[4638]: nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Dec 23 08:50:33 gokul nginx[4638]: nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Dec 23 08:50:34 gokul nginx[4638]: nginx: [emerg] still could not bind()
Dec 23 08:50:34 gokul systemd[1]: nginx.service: Control process exited, code=exited status=1
Dec 23 08:50:34 gokul systemd[1]: nginx.service: Failed with result 'exit-code'.
Dec 23 08:50:34 gokul systemd[1]: Failed to start A high performance web server and a reverse proxy server.

What could be a problem?