Having been absent in work

hello im a government employee..im assign in mobile alarm rescue systems department and worked nightshift..the time that ive been absent theirs an alarm call and theirs no one in the office..actually we are two who will be on duty that time and ive texted my co worker to come and informed him that i will be absent because i had loss bowel movement that time and i tried it hard to come to work because i always went back and forht to the cr..my co worker did not came that night..our chief is angry for what we've done..please help me write an explanation letter explaining to my boss everything without offending him..
thanks