Why is my 15 month old waking at night?

Saima

My little boy use to sleep through and be a good sleeper from the age of 11 month and now he is waking at different times of the night and refusing to go back to sleep. He would scream everytime we put him back into bed. I've been back at work for half a year now so I'm sure you understand how difficult it is for our family at the moment. Any advice on what it is please? Tia