hard jelly

I would try putting the whole open jar in a larger pot or bowl of hot water to see if it softens up (for a temporary fix and for immediate use). Not a long-term fix though.

Jelly or jam will soften and become liquid like when heated. If you put the jar in a saucepan with water, and heat it gently, you'd be able to remove it from the jar. At that point, you can thin it with a couple tablespoons of water. It may or may not re-jell as it cools- it depends on how much the pectin was damaged. It may be overly stiff due to excess pectin, or not enough juice/fruit, as well as overcooking. If it doesn't, it might still stay soft enough to spread. You can use it for the original purpose, as a glaze for meat (ought to be tasty on poultry or pork!), or as a glaze for things like poundcake.