When you dropped it, did it leak at all? I have had the same problem with my topbox nano from my subvod and my girlfriends topbox nano from her subox kit, as well as a subtank mini. I overfilled one, it leaked, didnt work. I replaced it and bought a new one as i thought it had just broke. Another one wasnt sealed properly, leaked, didnt work anymore so i returned it to the company for a new one as i had just purchased it the same day, i figured it had just broken again and i was unlucky. The last one, i dropped it, it leaked, and like all the rest of them it stopped working with the same problem you are describing, the resistance will not read on anything although everything seems to be in working condition i.e. 510 pin, coil, chimney, glass..... not until the last one did i notice the pattern of it leaking then not working properly. To solve the problem, i covered the driptip to seal the air in with my finger and blew air with my mouth into one of the airflow control holes as to come out the other side to get excess leakage out from the bottom near the 510. It worked for a few minutes (5 maybe) after that then stopped again. I then used a hairdryer in the same way, it worked even longer this time (10 minutes) then stopped working again. To finally solve the problem i had to take the whole tank apart, AFC ring and all to wipe the liquid from the base where the 510 pin makes contact with your coil. Be careful as taking the AFC ring off is tricky, theres youtube videos for disassembly of subtanks