Virtually every hawker centre has a few stalls open 24h, and there are 24h coffee shops all around town. Any specific dish you're looking for? None of these are exactly party places though, unless you bring enough buddies and buy beer
The best roti prata in Singapore is, oddly, not in Little India but up in Jalan Kayu near Sengkang. Thasevi is the original and they're open 24/7.