Napoleon abdicated on 6 April, 1814 having suffered huge military losses and being unable to raise additional conscripts from his own country to fight off the coalition's advance. But after returning to power in 1815, he somehow managed to raise 280,000 in a very short amount of time to take on the new coalition.
Where were these French troops coming from, and why weren't they available to Napoleon when he desperately trying to raise troops to defend Paris the year before?
The simple answer: he didn't. Much of the Napoleonic army was still under arms in early 1815, and those that had been disbanded were in many cases unhappy about it and ready to sign up again. Those that weren't were often unfit to serve in the first place. (Not that Napoleon's army at Waterloo was at all comparable in overall quality to the forces he'd lead before 1812.) I answered a very similar question in a little bit more detail last year, to which I'll refer you, and then I or others can answer any follow-ups you have.