How to manage administration tags

I’m having a little problem with managing the tags in the administrator panel. I used to be able to see all the users who had registered to my weblog (http://www.rosariocyc.com.ar) when the “users tag” dropped down. Now, not only it doesn’t display another options, but also the only function that allows me to do is to see the profile of one user (usually my own). I’m the administrator of the site. The other tags don’t drop down any more other features, “options tag” only displays “general options” and nothing else, the same happens with this tags: “dashboard”, “write”, “manage entries”, “links”, “presentation”, “plugins”, “import”, “polls”, “ratings” and “forum”
What could have happened? Can anyone help me? please i’m been invaded by spam users! and i can’t really know how many “real” users i have.
I’ve already tried deleting cookies and private data, because i thought that could be the reason for this to happen, but apparently the only thing i did was to delete all the passwords and else, but couldn’t fix this problem.
I’ll appreciate all the help and advice you can give me.
Thanks!
Verónica