This is not a straight forward situation, and is inherently fact dependent. Generally speaking, once a contract is signed, it is considered valid. You must remember, however that for any contract to be valid, there must be a "meeting of the minds," or an opportunity for negotiation of the terms. I'm not sure why (or how) you would sign a contract without seeing it or reading it, but such a contract would be of questionable validity. On the other hand, if the contract contained agreed upon terms and conditions, and both parties substantially performed their obligations under it, it would be difficult to invalidate it since it would be presumed (and usually stated in the body of the contract) that you have read and understand it.
Your best option for abrogating the contract would be if it was signed under duress, or if you can prove that the other party misrepresented the terms of the contract, and you relied upon those representations to your detriment. This would only work, however, with an "Executory" agreement; meaning that it is not an "integrated contract." In simple terms, if the contract reads, "this is the entire contract," then you would be limited to a breach of contract suit if the other party did not perform. You would be unable to explain to a court what was told to you, rather than what was actually in the contract, under the Parole Evidence Rule.
Finally, whether there is any value in pursuing legal action is entirely fact dependent, and would require more information than is given in the question. Such things is whether you had a chance to read the contract when you signed it, whether you were capable of understanding what you read (based upon your age, physical and emotional condition, education, etc.), and various other factors. The best approach would be to seek a short consultation with an attorney in you jurisdiction who can gather all the facts and give you a more specific answer.