Try the following commands in configuration mode:

Pix#config t

Pix(config)#ca zeroize rsa

Pix(config)#ca generate rsa key 1024

Pix(config)#ca save all

Pix(config)#write mem

Pix(config)#reload

That should fix the problem, but in order for the key to generate, you need to ensure that the PIX has a host name and domain name configured. If you need to do that too use these commands:

hostname xxxxxx

domain-name xxxxx.com