What happened, exactly, during the update process (or did you not see it, exactly?).
If it failed or rebooted during the update, it's probably toast, unless it was jailbroken and had the RP + CRP installed and diags still behaves.
Otherwise, serial access in diags may help, assuming you can get a shell, which I'm not too sure about.
(EDIT: what @j.p.s said
).