There was already a policy
Find its effective date, waiting provisions and claim instructions. An injury is not automatically payable, but the existing contract is the first one to investigate.
There was no policy
Discuss payment for current care with the clinic. Treat any insurance you buy now as a prospective purchase, not a rescue for the bill already incurred.
If coverage recently lapsed, ask the insurer about the actual status and consequences. Do not assume reinstatement changes the injury’s onset date. If you had another insurer before switching, keep both sets of documents so each company can explain its own position.
A clinic’s willingness to provide a treatment estimate does not establish coverage. Likewise, an insurer accepting an application does not approve the injury claim.