i replaced all of my computer components from my broken case to other case

hi all ,

i have a desktop

i changed my broken case to a new case like

i removed the HDD and the motherboard and the CD-rom ..etc and installed them to a new case

but when i press the switch button it works normally , and the screen display normally and there are no beeps , but after like 20 second my computer turns off automatically ! ,

what may cause this issue ?