mongod: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/mongod - from package: mongodb-server

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mongod - from package: mongodb-server

You may install the required package with command apt-get install {package}

Error in other languages:
mongod: bevel nie gevind nie
mongod: comandă negăsită
mongod: comando não encontrado
mongod: commande introuvable
mongod: command not found
mongod: käsku ei ole
mongod: không tìm thấy lệnh
mongod: komanda nerasta
mongod: Kommando nicht gefunden.
mongod: kommandot finns inte
mongod: komut yok
mongod: nie znaleziono polecenia
mongod: níor aimsíodh an t-ordú
mongod: no se encontró la orden
mongod: no s'ha trobat l'ordre
mongod: opdracht niet gevonden
mongod: parancs nem található
mongod: perintah tidak ditemukan
mongod: príkaz nenájdený
mongod: příkaz nenalezen
mongod: команда не найдена
mongod: командата не е открита
mongod: 命令找不到
mongod: コマンドが見つかりません