As is always the case on the "What will happen to me?" questions, It Depends.
If the attorney who wants your testimony does not subpoena you, then nothing will happen if you refuse to testify. However, if they do serve you with a subpoena, you are being ordered by the court to appear and give testimony. If you refuse, the attorney has two options: (1) do nothing, or (2) have you attached. It sounds like you are an important witness to this case, so the do nothing option may be unlikely.
Option (2) is a fancy way of saying that the attorney asks the judge to have you arrested and and brought to court. Once you've been brought to court, the judge will order you to take the stand and testify.
At that point, if you refuse to testify, the judge can hold you in contempt of court, which is punishable by a fine, but the judge will most likely order you to be taken into custody. He/she can hold you in the county jail for up to 180 days (6 months) before he/she has to let you go. But in this situation, we say that you have the keys to the cell in your own hand, because you will be released as soon as you comply with the Court's order, i.e. when you testify.
If you don't want to testify, they cannot physically force you to speak, but they do have ways of helping you change your mind.
Please understand that I am merely answering a question on a public forum, and that I am not giving you any legal advice. This answer also does not create an attorney-client relationship, and nothing said on this website is confidential or covered by privilege. You should always consult an experienced attorney in your area before acting on anything written here.