He spaced out for a while and didn't hear what the teacher was saying.

他一时走神，没有听到老师所说的话。

space out是比较口语的说法，意思是一时走神，一时失神。space out 也可指使(人)昏昏沉沉或头脑不清，如：

That medication totally spaced me out.

那药物让我完全神智不清。