Spent 30 years in the title insurance industry and this is more common than people realize. A title issue discovered after closing is exactly why title insurance exists, and your first call should be to your title insurance company to file a claim. Under your owner policy, the insurer has a duty to defend against covered title defects and to either cure the defect or compensate you for your loss.
Common post-closing title defects include: undisclosed liens (tax liens, mechanic liens, judgment liens), errors in the legal description, forged documents in the chain of title, undisclosed easements, and boundary disputes. Each of these is typically covered by a standard ALTA owner policy, though there are exceptions listed in Schedule B of your policy.
The process for filing a title insurance claim is straightforward. Submit a written claim to the insurer with copies of all relevant documents. The insurer must acknowledge the claim within a reasonable time (many states require 15-30 days). They will then investigate and either cure the defect, negotiate a settlement, or litigate on your behalf. If the insurer denies your claim, you have the right to sue them for breach of contract and potentially bad faith.
One critical point: do not try to fix the title issue yourself before filing the claim. If you take action without the insurer's knowledge or consent, you may inadvertently void your coverage. Let the insurer manage the process -- that is what you paid the premium for.
Also worth noting: if the title defect was caused by an error in the title search (the title company missed a recorded lien or easement), you may have a separate negligence claim against the title company in addition to your insurance claim. These are distinct legal theories with potentially different damages, so consult with a real estate attorney who handles title disputes.