If no ticket holders match all five numbers and both Lucky Stars in a particular draw the jackpot rolls over and increases in value for the following draw. The maximum EuroMillions jackpot is €240 million, so although there is no limit to the number of rollovers allowed, the jackpot cannot go any higher than that.
Once the top prize reaches €240 million, funds which would normally boost it higher are added to the next tier in which there are winners, usually the Match 5 + 1 Lucky Star category. The jackpot can only stay at this level for four draws, then it must be won in the fifth draw at the jackpot cap. The Jackpot Cap page explains more about how it works.