Images that do not finish within 15 minutes, and videos that do not finish within 30 minutes, are cancelled and refunded automatically.
You may also see:
"Saving the result failed. Your credits were returned."
"This generation can no longer be finished. Your credits were returned."
What to do
For a new image, tap Reuse prompt on the failed card on the Studio page, then tap Generate.
For an edit or a video, set it up again on the Edit or Video tab. For video, a shorter length or the Fast model finishes sooner.
See the refund
Your credits are already back. Tap the Credits box at the bottom of the sidebar to open Plan & credits (on a phone, tap the menu button (≡) at the top right first). Under Credit history, look for the line starting "Refund:".