The first issue to assess is whether your teen has withdrawn from his/her previous social life or if he/she is having difficulty making friends in the first place.
If your teen did have some friends (regardless of how vast or involved his/her social life was), but is no longer responsive when they call and has socially withdrawn -- this behavior is a warning sign that something is going on with your teen. In this type of case, examine what's been going in your teen's world. Is there a major family issue that can be causing emotional turmoil in your teen (e.g. divorce, abuse, or alcoholism)? If not, perhaps your teen has suffered a private trauma (e.g., getting bullied). So do talk to your teen about why he/she has chosen to stop socializing. If your teen isn't opening up to you and the behavior continues, you might want to enlist another trusted adult in your teen's life, such as a clergyman, favorite teacher, or even a professional therapist.
If your teen is having difficulty making friends, whether due to shyness or an inability to connect with his/her peers (often highly intelligent, highly emotional, or teens with learning disabilities have challenges relating to their peers), you can offer him/her some help in making friends. First, encourage your teen to get involved in activities he/she does like. Model good social behavior for your teen; ask yourself: how welcoming is your home? Do you have your friends over so your teen can learn some socialization? However, don't pressure your teen to all of a sudden run for class president. Your teen might be naturally solitary, which is fine to a degree. So be realistic in your expectations regarding what types of changes you want to see in his/her social life.