@Hang Wong
In this situation where you are the only admin of the tenant and if you have no access to the tenant due to "forgot password" or for any other reason, then you will have to contact Microsoft support team.
You will have to open a case with data protection team. You will have to prove your identity first and prove that you are the tenant owner.
Post this they will be able to either reset the admin password for your account or create a new admin and will share the credentials with you.
Let me know if you have any further questions.
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