Heat alone can vanish some kinds of normal, non-disappearing ink. Carefully warm up the paper over a stable candle and see if it works for the particular ink used.
Another option is to scan the document at high resolution (before or after other treatments such as heat or chemicals), and examine it in image editing software. With some basic knowledge of image filtering and level adjustments, you can often extract information that was hidden before—the idea is to find the subtle difference between the covered text and the plain paper, that you can adjust to increase the contrast between them.
Let's say the hidden text is just slightly darker. If you pick a brightness level that is between the two, and adjust the image to make all brightness values above that level white and all brightness levels below that value black, the original text can be discerned. Differences in color or any aspect could be the trick for restoring enough of the original text to read it.
I have done something quite similar with old receipts that had lost their ink by sitting in the car on hot days for too long. After scanning at high resolution, simple adjustment of the levels, contrast, brightness, and so on, restored the image enough for me to see the original text, despite it being so faint that I could not read it from the original paper even with a magnifying glass.