HELP!! My son is waking up screaming

My 16 month old has started waking up hysterical within a couple of hours of going to bed. The first night he woke up aftee 30 mins and I spent an hour settling him. Tonight he woke up 1.5 hours after going to bed... 3 hours later and I'm just getting to bed now. The only time he wasn't screaming hysterically was when i was holding his hand or shushing him (shhh shhh shhh). I ended up having to lay on the floor for 2 hours because he would wake as soon as he thought I was gone. He is normally an absolute champion sleeper and it's not teething or getting sick as he's gone through all that previously and hasn't bothered him much. I have NEVER seen him like this. Any suggestions or have you delt with this yourself and can offer advise?