After my instant coffee (with chicory mix) powder got hard due to moisture I decided to heat the hardened coffee in a pan, and it got soft stuck to the pan like chocolate, and even had strings when I broke it (something like when you pull a caramel bar). I expect it to form a powder on heating, instead of turning into candy. Was it because of the water content?
This also happens with common unsweetened instant coffee (without chicory) if you allow it to cake and then try to heat in a pan or any other kind of heat that is concentrated on a specific spot or surface.
Instant coffee (a.k.a. soluble coffee) is made by spray-drying brewed coffee, and not by finely grinding coffee grains. You are not dealing with a moist powder that can be dried by local application of heat, like when you have wet flour or wet coffee grounds.
The coffee was partially dissolved by the moisture but it lacks the crystalline quality of other solutions that naturally form grains or crystals - like salt or sugar - it then becomes an amorphous mass and when you try to heat it up, it melts instead of crystalizing. This will also happen if you try to heat it up in the oven.
You can still use it as is - it will take longer to dissolve and need more stirring than you usual powder. The only way to fully recover it is to replicate the process used to make it - re-dissolve it and spray-dry (which is very difficult to do in a domestic setting)