Actually it doesnt really say much at all. Is this a retail Xbox? Is this a flashed console? Is this a Jtag/RGH? Have you modded the game? If its flashed how did you burn the disc, what discs did you use? If its a retail what condition is the disc in..................
Are you referring to when you use a modded save? I know that if you use a save and mod the level too high, and try to continue the save at a later time that the game does not let you do any more missions. If this is the case, then you have to mod your save to a lower level and then the game will let you play missions again. If this isn't the answer you're looking for, will need more details as Bullet asked above.
if nothing helps mate iv posted a gamesave with everything unlocked and all skills, perks etc maxed. iv posted it up on xbox 360 gamesave