loop() either stops working, or lcd stops working | buttons don't work
The buttons will send a LOW signal when pressed if they are set with pinMode INPUT_PULLUP. 

The issue with currentNumber not displaying is that after you clear the screen with the 'for' loop, you need to reset the cursor to position 0,1 with lcd.setCursor(0,1); . Otherwise, the numbers are printing out of view.

If I add lcd.setCursor(0,1); just above each line with lcd.print(currentNumber); , I think the code has the behavior you're looking for.

Edit from the person who asked the question:
 You also have to add a delay of around 150-200, or else one push will count as many.