1. Communicate with the hospital: First, you can communicate with the hospital to find out why the certificate cannot be issued. Because your illness is not serious enough, or the hospital thinks you don't need to call in sick. If you have any objection to the hospital's decision, you can try to communicate with the hospital leaders or relevant departments.

2. Seek the help of other doctors: If you can't convince the hospital to issue a certificate, you can seek the help of other doctors. You can consult doctors in other hospitals to see if they are willing to give you a certificate. Please note that you need to pay extra medical expenses.

3. Provide other certificates: If you can't provide the hospital certificate, you can try to provide other certificates, such as medical records, diagnosis certificates and medical invoices. These certificates can prove that you have been sick or treated, and help you to ask for sick leave.

4. Communicate with employers: Finally, you can communicate with employers to see if they are willing to accept other certificates. If your employer doesn't agree, you can try to explain your situation and provide other certificates to support your leave application.