The title of this post is just as relevant to real life as it is to D&D. We, as a Western culture, have been ingrained with the notion that problems exist to be solved. Most of the movies and books we consume are focused around the main character overcoming the problems in front of him/her in order to reach that happy ending. Life doesn’t work that way. Neither does D&D.
Sometimes things just happen.
Despite all of your planning as a DM, you are designing in a vacuum. You design situations and encounters to be interesting, and offer possibility. You have some idea what the players might do, but you ultimately have no control over it. Events may not unfold as you anticipated, and fate (the dice) can be a cruel mistress.
When something happens that you didn’t intend, especially if it’s something negative, it’s your job to roll with it. You will be tempted to adjust the “reality” of the situation in order to ease the players’ burden. Don’t give in to that temptation. All that does is lessens the experience for them. Yes, it might be difficult – and they now have new obstacles to face – but dealing with obstacles is what makes the game what it is. Your players will of course remember the battle that went south, but they’ll remember the interesting choices their new predicament presented – and how the story evolved from those choices – even more.
There is rarely one “right” decision for any given situation. Sometimes you just have to let things happen. When it’s over, pick up and move on. That’s what makes D&D so engaging. It truly is a story where anything can happen. Let the players become heroes. Don’t try to mold them into your idea of a hero. If the players know that they will only ever face problems with a single, clear solution, the experience suffers. It loses impact and tension. It loses drama.
Not every problem has a solution, and that’s ok.