Think I messed up last session, need course correction advice
So for context last session my players were rescuing a player kidnapped in the heart of the enemy nation in my world. The pc was kept captive and tortured due to being soul-bonded to a young dragon who the enemy nation needed for nefarious purposes.
The session proceeded with the pcs killing a longtime villain who they’d had a grudge against for a while, a mid-tier commander within the military hierarchy. In my poor judgement and lack of extensive enough prep I fell back on improv skills later into the night, due mainly to some really good rolls and smart strategizing on the players behalf.
As a whole I envisioned my players succeeding but needed the enemy nation to have made their own victory to maintain the threat level. And so as it was getting later into the night I foolishly ended the session on no uncertain terms that the bbeg (an ancient blue dragon) came clawing up from underneath their boat.
Due to being short on time we ended there, which left a very large unambiguous problem for the party in which they most certainly would be unable to solve by normal means.
I feel I screwed up really bad, I have no idea what they may be planning, and also struggle to prep for anything without seeming either too railroad-y or too forgiving.
I really need advice on this and am open to any questions, thanks.