It is hard to guess what kind of a “crash” happened and why did it require the drastic roll back of the site.
As far as themes go — they are normally just folders of code (if not going into complicated ones with page builders and such).
- If that code survived somewhere in your case — it can be easily moved to a different (version) of the site.
- If it was erased — the time and costs for attempting to restore it would likely exceed what is reasonable.
As opposed to theme, the content lives entirely in database, not filesystem. Again — it’s either survived in some form or it didn’t, depending on what went down.
In a nutshell — enumerate the work you did, inquire if it’s still preserved in some form.
PS Usual lessons — use version control and such. 🙂