Simple question Simple answer, just saw it before i was about to ask my simple "newbie question".
Go to your xbox hard drive and find the file that you placed there to mod the box. DELETE it. Then reboot your xbox to make sure your softmod is gone, you can use the backup option before deleting it if you wish but you don't need to. After the soft mod is gone it won't let you on live? So go to settings, then network settings and change your ip configuration to automatic(it should be set on manual because your softmod (linux by any chance?) won't allow you to use xblive. let me know if this didn't work. btw if you don't switch your settings to automatic your ip adress will be different so you will get an error msg saying xblive not responding. please try again later.