Problem

Q. I enabled Magnifier on login screen , then it didn’t worked correctly and covered right half of my screen !
Now I cannot login and cannot disable magnifier and if I restart my computer Tthe magnifier will start again. Here is How I solved it.

1. Open terminal type

$ sudo nano gnome-mag.desktop

Give your administrator password, when asked.
2. Then search for the line

StartupNotify=true

and change to

StartupNotify=false

3. Save the file by hitting Ctrl+O followed with single Enter.

4. To Exit Hit Ctrl+X

This will reset the magnifier disabled, no removal of software required. And next time if you want magnifier be enabled, just enable it from the accessibility options.

Just to make sure it’s really working, try restarting.