Here is a classic case of being dependent upon a GUI
and not understanding the actual underlying technology.
When the GUI ceases to function, the user is dead in the water
and does even know where to find a paddle or what one looks like.
By installing 8i you've probably created a new ORACLE_HOME;
which no longer contains the previous customizations.
You need to learn how SQL*Net works so that you can get TOAD
"talking" to all your DBs again. The fix is trivial if you know what
SQL*Net depends upon for successful operation.
Which OS & version?
(Some flavor of Windoze, I suspect.)
What specific version of Oracle?