How could a groomer have "lost" your dog? Did the dog run away? Was the groomer negligent?
You may be able to recover on a bailment or negligent entrustment theory if you can show negligence by the groomer in some way (either by allowing the dog to escape by failing to take proper precautions or by doing something that caused the loss of the dog). What I would do is write her a letter, recite the facts that led up to the loss of the dog, your recovery and the vet's care. Provide copies of the bills and ask for reimbursement. Give the groomer time to respond - no more than 30 days. Does she have insurance? If so, then ask for the information so you can file a claim if the amount exceeds her deductible.
If the groomer does not respond, then seek to sue him/her in small claims in the magisterial district in which her business is located. I assume your claim is less than $8,000. If it is over that, then you will definitely want to seek out a general civil litigation lawyer. If nothing else, pay the lawyer for 30 minutes of his/her time just to go over the information with you and apprise you of your rights. Is your dog okay now? Can he walk? I don't know what the dog is worth, but the most that you could recover would be the value of the dog and maybe the vet bills.
If the groomer is a sole proprietor or sole proprietor doing business under an assumed name, sue the individual. If the business is a corporation or LLC, you sue the corporation/LLC and serve the registered agent or officer (which may be the groomer).
You may have to get another professional groomer to come to court and testify for you as to what proper procedure is for dog groomers to protect the animal. Your expert dog groomer will then have to testify that what was done here was negligent in that the groomer did something wrong or failed to take precautions so as to allow the dog to escape.
