I have a problem with my Pix 506E : I can't connect by telnet session. Is there any option to reactivate it from the PDM ?
Yes, there is a way to enable Telnet access via PDM-
Cofniguration -> System Properties -> Administration -> Telnet
Here you can add the host IPs from where you can telnet and specify the interface where these clients are located.
Note: You cannot telnet to PIX firewall from outside/lowest security level interface.