The episode will be called Grave Danger, and CSI Files has the details, but below is a small summary from Monsters & Critics:

In the
episode, main character Nick Stokes is kidnapped by someone with an apparent grudge against the CSI team. By the time he wakes up again, he’s trapped in a coffin, together with his loaded handgun and a tape recorder with a message telling him it’s probably best to kill himself, as he’s going to die anyway. There’s also a webcam in the coffin, which transmits an image back to the CSI lab…

The episode is the season finale and will air on Thursday, May 19th. [Thanks to Christian for the heads up]