Choose Sign in, then the reset link, and enter the email address you registered with. A reset link arrives by email and is good for a short time — use it promptly, and request a fresh one if it has expired rather than trying an old one twice.
If nothing arrives, the usual cause is that the account is under a different address than the one you tried. A second address you use for shopping, a work address, an old provider. Try the others before assuming something is broken.
The other common cause is that there is no account. If you have only ever booked as a guest, there is nothing to reset — you have appointments but not an account. Register with the same address you booked under and your existing appointments attach to it.
Reset emails sometimes land in spam or in a promotions tab. Check both before requesting a third one.