Question Pc shutdown randomly and cmos restart

[Redwan515]

I had a mini itx pc (acer veriton) and i wanted to change the case so i can add a gpu in the future.
So i did it and now my pc keeps shutting down randomly and won't turn on again until i unplug the power for 20 seconds and replug it
And whenever i start the pc again it does 2 beeps and say that the cmos forgot time/date and say the chassis has been opened
So i changed cmos battery and psu and it's still happening
Im pretty sure There's no overheating
Please help me

 

[Redwan515]

Btw the errors are
Cmos checksum error
And cmos date error
And warning your chase has been opened
Theres a little plastic thingy in cmos clear and i removed it by accident i mean maybe that's the case
Im scarred if no one will help me i might have to buy a new motherboard