While a long rest will restore all of your hit points, it will not necessarily replenish all of your spent Hit Dice. Once a long rest is completed, you will regain spent hit dice up to a number equal to half of your total hit dice (rounded up).
For example, if you're a 9th-level character (meaning you have 9 total hit dice), you can regain up to 5 spent hit dice upon completing a long rest.
This mechanic reinforces the strategic aspect of hit dice usage. If you've spent a significant number of hit dice during short rests, a long rest won't instantly replenish all of them. This means you may go into the next day with fewer hit dice to spend, making careful management of this resource critical, especially during long adventures or dungeons where opportunities for long rests might be limited.