Invisible Ink, How can I make the combination visible again?

You can try a black light or a UV light that can be used to check money to see if bills are fake.

The part that had the ink may not be as reflective.

Heat may be able to bring it back also as Ron stated. A candle may work but if it gets too hot too fast it can either burn the paper or totally darken the area where the flame is. You may try an iron set at low heat and make sure you do not hold it still, moving it across and off the paper to see it you are getting any results. If not slowly increase the heat. I have also used a standard light bulb for that. If the paper is held against it when it is cold and you first turn it on it will warm up with the bulb. Again if it gets too warm it will turn the paper brown and it may be lost so keep an eye in it. There are some that seem hopeless but if all else fails do let the paper turn brown and it may show in a different shade but that it defiantly a one time shot.

If it is mechanical and preset, aside from going through all the possibilities you may be able to contact the manufacturer with the serial number and a sales agreement if you have one, they may be able to help you. There is also the option of a locksmith if you consider it or the contents to be worth the expense.

Usually warming it over a small flame like a candle or match does the trick, obviously at a distance where you're not going to set the thing alight.

If it's a mechanical lock, then you can have as many goes as you like.

If it's electronic, there is usually a key override to that, so if you have the key - it will probably open with that.

I can tell you more, but you can PM me rather than tell the world.

How can we be sure it's your safe and you just haven't stumbled accross this piece of paper....

piss on it and itll do just fine