The other answers mentioned _what_ happened. In terms of next steps:
(1) Generally, receipts should be scanned (and perhaps re-printed, depending on your needs) for any sort of conservation effort. The heat from scanning (not in any modern tech, just as an FYI so you can decide for yourself) can cause the same degredation. It doesn't necessarily take a long time either. If you ever order a pizza where the driver tucked the receipt in the box you'll also notice that chunk of the receipt is illegible.
(2) If it's partially visible in its current state, you can probably get away with increasing the contrast and applying a decent OCR to the result. You'll get some hallucinations, but if you fill in the most likely characters conforming to the grammar of a typical receipt you're likely to be in the right ballpark (if you need a particular warranty code or something you might have to manually try a few options for particularlly blurry letters). Pair that with a photo of the receipt in its current state for any distrusting shopkeeps.
(1) Generally, receipts should be scanned (and perhaps re-printed, depending on your needs) for any sort of conservation effort. The heat from scanning (not in any modern tech, just as an FYI so you can decide for yourself) can cause the same degredation. It doesn't necessarily take a long time either. If you ever order a pizza where the driver tucked the receipt in the box you'll also notice that chunk of the receipt is illegible.
(2) If it's partially visible in its current state, you can probably get away with increasing the contrast and applying a decent OCR to the result. You'll get some hallucinations, but if you fill in the most likely characters conforming to the grammar of a typical receipt you're likely to be in the right ballpark (if you need a particular warranty code or something you might have to manually try a few options for particularlly blurry letters). Pair that with a photo of the receipt in its current state for any distrusting shopkeeps.