why is my 640xl phone not accepting my SIM card?

Till the previous night the phone was accepting my SIM and in the morning, the phone showed "Data connection error". The phone behaved as if it has no Sim card in it. I tried soft reset and hard reset. But of no use. The sim is working in other phones. So, took the phone to service centre where a new sim was inserted and tested. The new sim worked. But not my sim. Later the memory card also was not accepted by the phone. Last year, the front camera stopped working.
What could be the problem? I really want to buy a new phone, but no money. I have to make this phone work.

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