I think a better question would be: Do I want to allow the player characters to reform the planet?

Ultimately, it can all be handwaved. Applying enough energy to squeeze something together, or even using a nanite fog and time to make an in-situ 3D printer, and you'd be able to reform a planet. If you have detailed enough raw data of the planet - layers, compositions, tectonic plates, and so on - and you might be able to reform the planet.

You would also need enough info to synthesize atmospheric compositions, radiologically active isotopes, or even seed the myriads of biotopes with life - perhaps if there was some form of DNA library (the seed vaults of Svalbard would be woefully inadequate in this case - you'd need something with way more detail) you might even technologically reincarnate lost lifeforms.

If you lack a precise enough definition of the planet, you might be able to make a planet. I.e. its gross physical description might be the same or similar, but it'd be like the difference between Khal Drogo and Khal Moro.

Building a giant sphere-like object means you need more mass from somewhere to form the object itself - and that's before you start reforming the planet. That means you're approaching ringworlds (like in Larry Niven's eponymous novel, or as in the halos of HALO), and you're likely cannibalizing other planets (note: plural!) in your system to get enough mass to even go there.

You could, of course, just wait for the system to settle again. A smashed planet will eventually form another planet. It's just that it'd be the same planet only in a metaphorical sense - even if you somehow could seed the planet with the same biosphere as it was at the moment of impact, the environment would be different.

It is, essentially, a question only you and your players and their characters can answer. Philosophically, a man can never step in the same river twice (to paraphrase Heraclitus), and in the same way you might be able to do the physical reforming of a planet - but it won't be the same planet as the player characters had. They have changed, and the planet has changed as well. Essentially, some things cannot be unbroken, remade, or reforged. A planet is likely one of those things.

Super-science might give the option of time travel to mitigate or change the conditions that led to the smashed planet in the first place. (A popular topic in movies. It usually doesn't end well.)