Presuming it is a Sun-branded data rack...
If you have a valid Service Contract in place,
you open a support call with your local support center.
http://www.sun.com/secure/contact/
If you don't have a service contract,
you open a support call and purchase a replacement key.
(same web link still applies)
The privileges of the service contract will decide how fast the key arrives.
This is a public discussion forum, not Sun techsupport.
Another alternative is that you could contact a local locksmith and they
may be able to custom-manufacture a key for you.