I would ask this on ELL but I honestly have no idea how to express this in English. Online translation give "brain short circuit", which sounds more like terminology than self-mockery. I am looking for a translation that fits here:
A: 你刚刚要说什么? (= what were you gonna say?)
B: 没什么, 我脑袋短路了. (= nothing, I just ____.)
Another imaginary situation:
Some guy solves a math problem using a very complex method, and some other person shows him that it can be done very simply, and he says: "Oh! Right! I ___(脑袋短路了)."