Hi hanqering,
It seems that the motherboard of the system has gone bad and needs replacement.
If the system is not covered under warranty and you are residing in US, you may contact Dell spares at 1-800-372-3355 to get the motherboard. If you are comfortable, you can replace it on your own or you may contact a local technician.
Please reply if you have any questions. I will be glad to assist further.