When you take it appart and open it, move the gear and yow will hear the pump, then press where the start button is, and hopefully it will restart.

I cannot open it in 2 pieces

I've quickly discovered these units are utter garbage, so many faults with them, no great wonder that as soon as you put Philips Sonicare Airfloss into Google search, the repairs/returns page is top result... I did enjoy my unit but I think its back to manual flossing as my primary/regular means instead of using in between Airfloss™

Mine had the same problem, pull very hard both sides, the upper where the reservoir is and the bottom at the battery. You will see a black sealing rubber junction, and then all of the mechanism. push the tiny little button and it will restart. Now turn it off and push it back. Good luck!