Napoleon abdicated conditionally on 4 April, the condition being that there would be a regency for his son. Caulaincourt and the marshals took those conditions to the allies and Alexander, who apparently had the final say, was about to agree when Marmont took his corps over to the allies which convinced Alexander that the army didn't support Napoleon, which was incorrect. Marmont's treason brought back the Bourbons. Marmont had been incluenced in his actions by Talleyrand who wanted the Bourbons back.
Caulaincourt and the marshals were furious with the outcome, especially with Marmont.
After this fiasco, Napoleon had no choice but to abdicate unconditionally.
Once again, I highly recommend Caulaincourt's memoirs.