You really haven't given us much to work with here either. What distro are you using? (brand and model number). What type of mouse is it? Is it PS/2 or USB? What are you urgently trying to do? (This will help us get you through the urgency as quickly as possible, perhaps dealing with the underlying problem later).
You should start with your profile. Click on "User CP" and then profile. Fill in anything that you don't mind others knowing (particularly your distro).
You may be able to change the focus with the <tab> key. This will be of little use if you do manage to log in graphically (I don't know many X shortcut keys).
A better approach would probably be to take this thing on in the text based world. By holding <ctrl> and <alt> and pushing F2 you should get a text based screen that should let you log in (you will probably need root access). type:
Code:
less /var/log/messages
<End Button>
This will give you your system log that you can look for mouse problems (mouse, psaux, Xwindows input devices, etc.). You can scroll with up and down arrows and push 'q' to quit and try another command. You can also try to "less XFree86.0.log".
You will probably end up needing to make changes to your /etc/X11/XF86Config file. You should probably use less and post here the input section that has to do with your mouse.
Unable to set focus to the user name field in login screen using keyboard alone
