If you cannot log in with your till login PIN, then your best option is to get your PIN reset. This can be performed by a System Administrator user on your site.
Once the System Administrator has logged in, they then need to go to Setup, Users, find and select the user within the list, then click Change..., type in a new PIN into both fields, Ok, Save.
You will then need to recreate till data (click to understand how to do this), and update your tills in order for them to pick up this update, to then be able to log into the till with your new PIN.