unfreeze: 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/unfreeze - from package: freeze

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

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