1st:
No, seriously... If your old thermal receipts have faded, they can sometimes be fixed by ironing them (no steam, just a hot iron or an incandescent lightbulb).
This only works if the thermal lettering has faded but the paper is still mostly white - The heat-sensitive layer where the original letters were is desensitized and the faded letters will not re-blacken, but the rest of the receipt will, giving you a negative copy of the receipt.
Note that this doesn't work for all thermal printer receipts, it depends on the type of paper used in the machines (the catalina coupons at most supermarkets (they print on thermal paper with red strips on the top/bottom) work, but the register tape at the supermarket I used to work at wouldn't - not sure why.
2nd:
I have had those thermal receipts end up blank after a few months as well.
I scanned one of the now blank looking receipts into photoshop. I was able to play with the levels adjuster and most of the receipt was readable. I printed out the new one and stapled it to my old faded one. Fortunately I havent needed to return anything recently......knock on wood.....
3rd:
've had this problem more than once, and with a bit of luck and some intuition found a method to restore the thermal "image".
By gently heating the receipt at a distance with a hair dryer, I have been able to cause the original images to reappear. Note that too much heat will cause the entire receipt to darken, so you may want to test temperatures/distances on a blank area, such as the corner of the receipt.
Generally, a hair dryer on medium heat at a distance of 12" or so, waved slowly back and forth, can restore the original thermal image in under a minute. Your mileage may vary.