Nozawa Onsen is up by the Northern Alps, north of the city of Nagano. You'll take Keikyu train from HND to Shinagawa, transfer to JR Yamanote line (commute train) and head to Tokyou station, then hop on bullet train from Tokyo to Nagano, then finally change to another JR train (local train that's probably pretty slow) which takes you to Nozawa Onsen. Even though the bulk of the distance will be covered by the bullet train which does one shot from Tokyo to Nagano in little >1hr, I'd imagine the whole trip from HND to Nozawa Onsen will still take you 4-5hrs. I don't know what time your flight arrives... if it arrives later than ~4pm, then traveling to Nozawa on the same day may not be a great idea. If you're staying at a ryokan, a lot of ryokans will not serve you dinner beyond 8pm.