Chris is right-on in his comments to you re: your register key not working. But I think we could help you more if we knew what is 'not-working'. Can you see anything out of order on your instrument? Is the pad beginning to come apart? Can you blow air through the register hole? Is the key itself moving readily up and down? Does the register key leak air when you close the thumb and three finger holes (with your left hand) then sealing the bottom of the clarinet 'section' with your right hand and attempting to blow through this section? I believe if we had more specifics on the problem Chris or I (and others, no doubt) could give you a "what to do" to do a fix on your instrument.
FROM CLARINET-NOW.COM
Thanks Glendon,
Absolutely right, making sure the clarinet is working is extremely important and will cover all bases. Thanks for the remarks.