yesterday i was so busy that i couldn't take a rest

-> Yesterday I was so busy that I couldn't take a rest.

i am a computer assistant in the school.

-> I am a computer assistant in the school.

but yesterday suddenly native teacher's computer was not working

-> But suddenly yesterday a native teacher's computer was not working.

so i tried to fix the computer but i didn't know why the computer is not working

-> So I tried to fix the computer but I don't know why the computer is not working.

so i tried to format the computer but the teacher want to back up her files

-> So I tried to format the computer but the teacher wants to back up her files.

because the files are needed to teaching

-> Because the files are needed in teaching.

so i took long time for fixing the computer

-> So it took a long time for me in fixing the computer.

yesterday was very busy day

-> Yesterday was a very busy day.