Open Question: How can I get my 16 month old to sleep?

Just under a week ago my little boy stopped sleeping through the night. I'm not talking about just waking up once. I'm talking about waking up any time from every half hour to 2 hours max. His diet hasn't changed (and its reasonably healthy), His routine hasn't changed. He's recently had his back top molars come through but that wouldn't cause sleepless nights for nearly a week would it? He'll only sleep if i pat his chest til he sleeps. Then he'll wake up and realise we're not there so screams and screams til we go in with him and pat him back to sleep. the only way i can get a half decent sleep is if i'm cuddling him. We left a light on for him so it wasn't completely dark, had soft classical music playing but still he wakes and automatically stands up and screams. Even his daytime nap (which has never been a problem!!) is now a struggle. Having to pat him to sleep takes about 30-45 minutes. He still needs his naps because he's VERY tired but just wont sleep... Someone please help me??? He has his nap at 11 am everyday (just the one). We go for a walk 1-2 times a day (weather permiting) and more than not go to one of the 4 playgrounds close to us and play most of the day (except when i need to do the daily household chores, then he helps). His usual bedtime routine is tea, playtime with mum and dad, bath, warm milk and a story read to him then mum and dad tuck him in. Hasn't changed... Like i said, he's never done this before so i'm wondering what could have sparked it...