If the red light flashes, your pass is invalid. It has either been cancelled or it has expired or you are travelling on the wrong bus. The driver can print out the reason for this for you. You cannot travel with an invalid pass, so will have to pay the appropriate fare or board the correct bus. To renew your travel pass, apply on the Post16 transport page.
If the pass is not recognised and there is no light or sound from the machine this means the chip in the card has been broken. Smartcards are robust if taken care of. However, just like a bank card, if a smartcard is bent too far or cut into the chip inside will break. If your pass has been broken you cannot travel unless you pay the appropriate fare. Visit Replacement passes for more information.