Optiplex 780 sff problem

Jan 27, 2019
Hello guys i repaired my psu ob my optiplex 780 sff and got a case of optiplex 760 sff Also front power button is from optiplex 760 sff and with the fan including the speaker i put my optiplex 780 sff motherboard on the 760 case and conecting 760 io front power buttn and the fan i said now my problem starts i tried to boot it up but when pressing power button it wont start and its orange i could boot my pc by holding psu button and if i relased it it would shutdown i while i hold my psu button to powet ut up it came a message alert system battery volda is low but i manage to fix it abd wont appear anymore also while i prees f5 for diagnostician it said a warn that my hdd is not working but my hdd powers up and spin i need help guys also it says no boot device avaible could it be the psu ? But its working thine could it be the 760 sff io front? Maybe i should put my 780 sff io front help ... sorry for my English
Here's the Optiplex 780 manual.
I would try swapping the front I/O panel from the 780. You should be able to make that swap work. The 780 is basicallly a 760 with DDR3 support added.
If you installed the new motherboard battery the BIOS would be rest to default. The Opti 780 has a UEFI BIOS. There may have been some legacy mode settings turned on, that got lost during the reset. I would get your hardware all back to the 780 configuration and ask at Dell Community Forum for help with that specific sytem. They don't get involved with warranty or tech support charges like Dell Support does.