Hi, this is likely a hardware fault and can only be repaired by a nokia care point/centre. You say that the guy from the Nokia service centre told you the could not repair it, I would like to know why he said so. Are your phone out of warranty? Have you bought it in some other country? Did he tell you it had been physicali damaged or...?
If you or anyone else has not tried the hard reset, you could try that, but probably it won't fix it.
Pull your battery out - and your memorycard. Wait 5 mins and put only the battery back. try turn on normally. - If it don't work (I guess you've already tried that) - Press grenn call button + * (star button) + 3 (number 3) while powering on. Depress those keys all the time and don't let go before youre past the Nokia handshake logo screen.
You could try connecting to charger and see if it starts to charge and then try the hard reset.
If that won't fix it, you will need to get it repaired in some way.
You went of course to a official Nokia care point??