If you do not show up, you could be picked up and brought to court. You should call the person who subpoenaed you and discuss your situation. You will likely find that they will help you, if they can, miss as little work as possible. (For example, you may not have to appear until it is the day your testimony is needed. You may be notified of resets rather than be required to appear just to waste a day and learn of a reset.) It is best to be as cooperative as you can, and it is the law that you must appear when subpoenaed.
Well, there are a few ways to view this. First is that the way you have received service may not, and I underline may not, require you to appear. Here is why, just because someone sent a letter to you in the mail can they prove that you have received it? Can they prove that you have read it? Probably not, right? So unless you call the number the printed on the subpoena and identify your self and acknowledge receipt of the letter, the party who sent you the subpoena can't actually prove you received it and therefor they will have a difficult time convincing the Judge you were properly served.
The other side is your civic duty. You did receive the subpoena and may want to place yourself on call. Think of it this way, if you were ever in trouble and needed a witness to appear, you would certainly want their cooperation. Remember more than 95% of the cases settle prior to trial, so chances are you will never even be called into court to testify. My suggestion, place yourself on call.
What will happen depends upon whether you have acknowledged receiving the subpoena. The District Attorney's office typically sends out subpoenas by mail. They simply do not have enough resources to personally serve each witness to a case. It is the DA's responsibility to follow up with each witness. If the victim really needs you and you have a relationship with this person, you may want to call the DA's office and place yourself on call.
If you decide that you don't want to show up and you haven't acknowledged receipt of the subpoena, nothing will happen. If the DA wants or needs you to make their case, they will call you and ask you to come in, which you do not have to do unless you have been personally served. If they really want or need you they will serve you personally with a subpoena.
If you call and acknowledge the subpoena, the DA will place you on call and try very hard to limit the impact to you personally by calling you only when they need you. If they must personally serve you, and they believe you are uncooperative, they may have you ordered back to court every time there is a continuance or make you wait at the courthouse until it is your turn to testify. The difference could be a couple of hours inconvenience versus a whole day or having to return day to day. It is best to cooperate especially since most cases resolve short of trial.