Admin panel doesn't work

Hello.
Can't access at site.ru/manager/
An error 404.
In the memory of my browser there is such link: site.ru/manager/index.php?a=30&, I using it,
get to admin panel, but old login and password are wrong! And doesn't want to restore them.
I entered without any problems earlier.
I didn't do any operations with files.
I will be glad to any practical advice.
Thanks.

Did you make any changes to the server and/or website recently (ex: move servers, upgrade php versions)?