There are a couple reasons this could have happened. Either you've run out your 90 day temporary serial number, or you need to enter your Runtime Key along with your permanent serial after registering.
Your 90 day temporary serial number has expired. To check this, you can look at your serial number. If it is not paired with a Runtime key and the fourth digit in the numerical portion of the serial is a 3, then it is a temporary license only good for 90 days. Not to worry, though, you can simply head to register.maxon.net
and register with us to be sent your permanent license.
You are not entering the Runtime key along with your permanent serial number. Since the full serial and the Runtime are tied together, they must be entered at the same time; if the Runtime is entered without the Full, the program won't know what version of the software you're using, and if the Full is entered without the Runtime, Cinema 4D assumes the Runtime is 0 and throws an Expired Serial error. To avoid this, make sure you copy the full line of text for both your full serial and Runtime key into Cinema 4D (start your selection at the word "Cinema" and end after the last >> in the Runtime key, then copy and paste all those lines of text into the installer, or the registration window that comes up after your expired serial error).