Do a search, somebody asked this not long ago.
It's a safety feature if you've been trying to have both windows up/down at the same level and been moving them a bit at a time, it thinks a kid is messing with the buttons and cuts in to stop them trapping their fingers.
With the windows all the way up or all the way down, hold the button up (if windows up) or down (if windows down) for around 5-10 seconds. This will reset it for you and they will then work as normal.
Please can people start using the search function. :coffee: