Unable to open my localhost/administrator page (no replies)

I'm not entirely sure whether this is a Joomla problem or a WAMP problem...

I'm very new to Joomla and WAMP.
I've been taking an online course on Joomla.
I recently installed WAMP - then Joomla.

It's been working fine.
Several times I've logged in to the new site I'm building, and made a few changes, while following along in the course I'm doing.
Today when I went to log in, I get a "404 Not Found" error when I type localhost/administrator in the address bar.
If I go to my 'c:wamp\www' directory and double click index.php - it asks me what I want to use to open this file.
I can see by the green "W" in my task bar that all services for WAMP, including PHP are up and running fine.

Any suggestions as to how to fix this, and explanation as to what may have caused this?

Thanks,
Amber