OK, so it sounds like you're having 2 interrelated problems:
1. The physical hard drive is failing, which means you're in danger of losing the data stored on it. This is compounded by...
2. You encrypted some of the data on your drive with BitLocker, & apparently the part that stored your PIN for verification purposes is part of the data that's becoming corrupted.
I'm going to go out on a limb, though, & assume that you didn't bother contacting Microsoft... even though a) it's their software (so they would provide tech support for it), and b) had you finished registering with them, you could have signed up for the optional "escrow key" that would have allowed you to recover data even if you lost the PIN. That should be your first check.
Barring that, are you running Windows XP or Vista, & is the drive formatted as a FAT16/FAT32/ExFAT type drive? If your answer to both is "yes", then there's an app called "BitLocker To Go Reader" that allows read-only access to encrypted drives, & it might allow you to try recovering the data to another drive.
If all else files, you can take it to a tech support shop that can do bit-level recovery from a drive. They might be able to clone the data as-is from the drive (including the data in its encrypted form), & that might then allow you to access the data using your BitLocker PIN.
Unfortunately, this is not going to be an easy fix. Getting data from failing hard drives can be hard enough as it is without dealing with encrypted data. I hope you have a recent backup of your data stored somewhere...
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