howdy - and welcome!
first thing i would check is to make sure that the coil in the tank is seated properly - keep in mind there is a small silicone o ring under the coil (above the threads, below the 'fat' part of the coil head) - make sure the o ring is there, first... second, when you put the coil back in, be careful not to overtighten it (tighten it 'finger firm' - don't crank it down - over tightening can cause the o ring to deform and can be cause of leaks)
in regards to the mod (battery) does it power up at all (light blinking or anything)?