Dell Optiplex 9020 AIO can not start up

Upgraded ram 2 weeks ago. Then had a problem of stuck on Dell logo but just once a few days ago. Turned on whole night last night. Can not start up today. Tried to remove HDD and DVD, BIOS battery, replace ram, still not start. Organe blink very short time then white light and fans on for one second then off after plug in AC. when push power on button, repeat organe blink very short time then white light and fans on for one or two second then off till unplug. Measure 8 pin connector from PSU, got bot 12v and 5v. Is this a PSU or Mother board problem? Or others? Many thanks