Is the inability to wake up for Fajr prayer, despite keenness to do so, a punishment from Allah for a sin, and what should be done about it?

If you wake up at prayer time and feel too lazy to perform it, this is a great danger, and you must strive against yourself to perform the prayer at its appointed time, for intentionally abandoning prayer is one of the greatest sins. Seek Allah's help and supplicate to Him to protect you from the evil of your own soul and to grant you success. Repent to Him and seek His forgiveness for whatever might be causing you to be discouraged from performing the Fajr prayer.