Visit your veterinarian first to rule out a physical problem.  if there is no underlying illness, make sure you provide your pet with plenty of extra TLC (Tender Loving Care).  Many pets will experience a period of grieving over the loss of a companion.  You may want to consider getting a new pet when you are ready.  Your pet may or may not adjust to a new friend, it depends on his/her personality.  Your veterinarian can explain the process of introducing a new pet. You and your pet will need time to adjust to the change and your veterinarian can help ease this process.