Don't worry! This can happen when our parcels get a little cold during transit, but there's nothing wrong with the treatment once it's warmed back up again.
To do this, pop the cap back on the pipette and put it somewhere at room temperature, away from children and pets, for at least an hour (although it can take up to 24 hours to fully liquefy). Give the pipette a gentle shake every now and then, but be careful to keep the cap on!
There is a substance in the flea spot-on treatments that is used to dissolve and carry the active ingredients across the skin barrier. This chemical on its own has a freezing point of, and so solidifies at, about 18 degrees Centigrade, but with the ingredients this lowers that freezing point to just under 14 degrees Centigrade. Essentially what this means is that prolonged exposure to temperatures below 14 degrees Centigrade cause it to solidify.