I think Xandros proximity sensor is not working....i m not able wake up my phone��
I would clean the top part of the display first.
Remove any screen protectors., they can potentially interfere with proper operation.
If the phone has had service/been disassembled recently, something from that might be interfering with the sensor.
I've seen a report that spraying compressed air into the earpiece has fixed this issue on a Lumia 920. Can't say if it'll do any good on the 620, but if its onhand, its worth a try.
It could be outright broken, requiring service.
Proximity sensor has nothing to do with waking up your phone. Does your screen go off during a call when you bring it near your ears?
The proximity sensor has control over whether Glance and Double-Tap-to-Wake are active. Whenever the sensor tells the phone that its covered (whether by a hand, being in a pocket, or malfunctioning) glance will not activate nor will you be able to use double-tap-to-wake. Reasons being to save power and reduce accidental double-taps respectively.