Ya also faced that problem once in my life but it was not with a Sony Ericsson phone, it was with my Nokia....
Wat is the model number of your phone and when did you buy it, has it been too long??? Is it still with in warranty???
I think you should contact the people from whom you bought the phone, they might be able to help you out... And if you just bought the phone & if you are still with in the return policy then I suggest you send the phone back to them and request for a brand new exchange or just get your money back.