Its a lg model ht902-d0 when i put dvd in it wont recognize it and ejects . if i bang the player it starts to play

Possible there is faulty connectors or contacts on the main board or the mechanism and so when it is tapped the contacts hold on to make it fucntion.
So remove the covers, check and redo all contacts on the connectors- remove and replug- tap the baord and observe using a magnification glass to confirm.
use a good soldering iron to do a touch job on suspected points.