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