Had my enchanting table open. Went to close out the enchanting enterface by hitting the x and in the top right. I am playing in full screen. Hit the x for closing the program by mistake when it became unhidden. Closed the game without a prompt, " Are you sure you want to close?" etc.

Reopened the game to find my diamond pickaxe missing from my hotbar. Opened the enchanting table to find the diamond pickaxe was also missing from the table interface.

Cheers