You didn't get paid - simple as that. The same thing happened to people who had bought Confederate war bonds. The government responsible for paying them was defunct, and the Union wasn't about to subsidize the Confederacy's debts. This is specifically addressed in the 14th Amendment:

Section 4. The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.

Now, the former Confederate states did assume the obligation of paying pensions to aged veterans.