If "solution" refers to a way to solve a problem:
* Problem: This is the most direct opposite.
* Issue: Similar to "problem," but often implies a more complex or controversial issue.
* Dilemma: A situation with two or more difficult choices.
* Stalemate: A situation where progress is impossible.
* Deadlock: A situation where no progress is possible because of disagreement.
If "solution" refers to a liquid mixture:
* Solid: A substance with a fixed shape.
* Gas: A substance with no fixed shape or volume.
If "solution" refers to a chemical solution:
* Suspension: A mixture where particles are dispersed but not dissolved.
* Colloid: A mixture with particles larger than a solution but smaller than a suspension.
To give you a more precise antonym, please provide the context in which you are using the word "solution."