[previous response edited; deletion of unverified info]
Mike
Hello Dietmar,
options are limited to the suggestion by Nicolas - assuming passwords are available.
The switch should arp for a gateway soon enough. If no gateway is configured, try a link bounce to generate trap or syslog. Try a port scan/discovery security tool.
(Note the host management VLAN caveat)
If no IP management is available and console is disabled, management is not recoverable.
Investigating other options for your 800 turned out to be counterproductive.
* Management lockout is covered under neither warranty nor contract.
* I voided one of our lab warranties trying to locate a battery. (which I could then warn you not remove). No help here. I found none.
Mike