The email address you've been looking for is still with frontier mail or any other major search engine. It is not that there was a mistake made in verifying your registration, since they likely have not done that yet. It is just that the error was not found right away and you didn't register on time.
The first thing to check is if your frontier Yahoo email account has been suspended (which it should not be). If it has, you will have to discover a way to restore it. There are a few ways to do so, but the majority of them involve getting the individual's contact information. That typically involves asking friends and family members. You then try to call them on their phone number or email address to determine if you can find any confirmation that they have the frontier Yahoo email address registered. If they say yes, then you can go ahead and register your account.
If this doesn't work, you might have to resort to something called manual account recovery. This is one of the ways that Yahoo uses to attempt to get people to register their accounts. You won't necessarily be able to revive an account from a previous date, but you should at least get an idea of how things used to work. This way, should you ever run into the same problem; you will know exactly what to do. It's definitely better than having no idea how to get your account back, so you'll definitely want to take this route.
To find your password, you will need to know how to get to the “change" or “profile" page. Here, you'll get the option to modify your account's password, if you don't know one already. Clicking the “change" link next to your account's name will take you to a webpage where you can type in a new password for your account. Be certain you write down the new password so you can type it in later when you are online. Though Yahoo will offer you a new password by means of an email, it's always best to write it down in case your secret key gets lost (or worse, stolen).
Once you've typed in a new password and made sure that you wrote it down somewhere, click on “save changes now." A new window will open up with your own changes. At the upper right corner, you will see a line stating “Change Delorean Password." Click on it. This will take you to a page where you are able to change your old password into the new one. Do not forget that the new password needs to have a certain number of alpha-numeric characters; anything more than this is considered too simple for Yahoo to crack.
If everything was going smoothly until you went to type in your new password, you likely won't need to change it until a while later. However, if something happens and you need to change your password immediately, you might find that you can't do it over the Internet like you used to be able to. You will instead be directed to a login page where you'll realize that the new password does not work; instead, you'll be prompted to enter it on the spot.