*EP pin connect to Ground
You talking about the pad on the underside of the part?  Most likely that's there to dissipate heat.  People like to put parts in smaller and smaller packages but the heat that the silicon generates needs to get out somewhere.  If it's just some digital IC and it has other GND pins it will most likely work.  Should be fine if you're just messing around or prototyping, but fix it before you make more.  Just watch the temperature of the thing. 

If it's a mosfet or some other driver designed to dissipate heat then you may be in trouble.  You might be able to counter it by putting a little heat sink on top of it.   On the other hand if there's no other GND connection then you're in trouble.

Usually an IC is designed to operate in a range of temperatures so if you're just on the bench at 27C you have a good chance of being ok for now.

When you fix your board keep in mind that you probably don't want to just "connect" it to GND.  If you look in the back of the datasheet for it you'll probably see a recommended land pattern that shows how many vias it wants to GND and maybe even a requirement to have some copper fill connected to it on the back.