How do I make the writing reappear on a thermal paper recipt that has
lost its writing?

This solution was posted by "schwazie" on the Slashdot site:
  
"I'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."
  
( http://ask.slashdot.org/article.pl?sid=03/08/28/2043237&tid= )

Excellent comment by Jackburton!

And note that the National Archives of Australia smartly recommends
making photocopies of documents done on thermal paper:
http://www.naa.gov.au/recordkeeping/rkpubs/advices/advice38.html

Best regards,

Omnivorous-GA