cd player wont play cds?

Hi. Ive had my ford focus zetec for around 2 years, it is an 05 plate. It has a standard sony 6 cd multichanger headunit fitted, but it will no longer read any cd which i try to play. It will load the cd and sounds like its reading it but then displays 'check cd' message and ejects it. Ive tried countless amounts of cds and have the same results.