Admin note: split this post from a thread on an unrelated product.
We need a more information to help you.
What precisely was installed in Hyper-V?
Is it a Gateway, Management, or Both?
What version of code?
What precise login prompt do you not know the “admin” password for?
If this is urgent, I highly recommend opening a TAC case.
if it's a GW and it's connected to a management server then with CPRID or running script from the SmartConsole you can create a new user to access it and then change the admin password.
Also you can change the admin password directly:
Expert@HostName]# $CPDIR/bin/cprid_util -server <IP_address_of_Security_Gateway> -verbose rexec -rcmd /bin/clish -s -c 'set user admin newpass <NewPassword>'
This is the same command that can be executed from smart console and it uses cprid_util. Well even policy transfer happens through the same command on gateways from mgmt server.
No need to post the same message multiple times on the same thread.
For R77.x, you can use EmergenDisk to reset the password: https://support.checkpoint.com/results/sk/sk92663
Of course, R77.x releases are very much End of Support at this point.