Actually Ney lost the campaign/battel two days earlier when he failed to seizr the vital crossroads at Quatre Bras. Had he done so Wellington would not have been able to withdraw his army in good order to fight at Waterloo on the 18th.
Ney's forces reached the cross roads at about the same time as the advance guard of Netherlands/Nassau troops. Had he been more aggressive he could have swept them aside before the first British/Hanoveran units arrived (Picton's Division) to reinforce them.
Ironically two days later he would make up for this "cautious" behavious with his rash and impulsive Cavalry attacks against Wellington's centre on the ridge at Waterloo. He launched over a dozen unsupported Cavalry attacks against the Allied Infantry (26 Bns) formed in squares and failed to break them, losing both French Cavalry Corps in the process.
I personally think however that the main reason Napoleon "lost" at Waterloo was the weather. Had it not rained the day and night before the battle on June 18th he would have won. Because it did rain , the ground was too wet for him to move his Artillery around until the ground dried out. Napoleon was an Artillery General and more often than not his massed batteries won the day for him. The battle did not begin until after 11:00 am that day, not 8:00 when he wanted to start. Had the battlel begun three hours earlier , and even if everything else had happened as it had, the disaster at Hougoumont which kept Rielle's Corps pinned down on the left, the destruction of D'Erleon's Corp by Picton's Division and the Heavy Cavalry charge, Ney's impulsive actions, the battle still would have ended three hours earlier, before the Prussians arrived.
Wellington would have been forced to retreat and he would have needed at least two days to reorganize his shattered forces to fight again. By that time Napoleon would have been able to rest his army and turn and fight and probably beat Blucher on the 19th.
