Your use of the term "brain" is amusing but not useful, as it does not tell us what was really replaced.
Most folks would consider the CPU to be the "brain" of the PC, but I can think of no reason why a CPU by itself would be replaced because any issue that would damage it to the degree it needed replacing would also damage other components on the PC.
Regardless, if you just got this back from a repair facility and it's not working, we can do nothing about that. So, you would have to contact them to have them address the issue of getting it fixed.
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