In older versions, all you had to do to retrieve your Crystal Reports license key was go into Help > About. That screen would show the key and your registration number (if you registered the software). In more recent versions the key is no longer there. There is a license manager under the [Help] menu but it only shows you the first few and last few characters of your license key. I assume this was intended as a security measure.  However, if you need to reinstall Crystal Reports when you upgrade your hardware you might struggle a bit. Here are three other ways to find your key:

1.  Check your Email. Most installs are downloads and the key is Emailed to the person making the purchase. You might have received that Email or had it forwarded to you.
2.  Call SAP Sales. If you purchased it directly from SAP (the most common option these days) they should be able to look up your account and give you the key.
3. Or, my favorite, pull it from the registry. You will probably find it in this registry key:

HKEY_LOCAL_MACHINE/SOFTWARE/Wow6432Node/SAP BusinessObjects/
Suite XI 4.0/Crystal Reports/Keycodes/CR Dev

The registry key will contain the license key followed by an 8-digit numeric date, separated by a colon.