undoing changes to bashrc from terminal

You can't completely recover. But you can partially recover using set.

If you run set on the same terminal you'll be able to get a whole list of custom scripts and other environment variables set. And in that, it's upto to differentiate the ones that were part of .bashrc and others typed on that terminal. But you won't be able to recover the commands which used to be executed as part of bash login.

For the future you may consider using a version control system such as git or hg in order to save previous versions of files such as ~/.bashrc. Then if you happen to do a > rather than >> in the future you should be able to recover the file back to the point the last time you commited it to the version control.

An example of how to set this up for git would be:

cd ~
git init
git add ~/.bashrc
git commit -m "Added .bashrc to version control"
# Time goes by...
echo "export FOO=bar" >> ~/.bashrc # Added a new line
git commit -am "Added FOO to .bashrc"
# Time goes by...
echo "export SHEEP=lambs" > ~/.bashrc # Eeek! We've overwritten our file
# Version control to the rescue
git checkout ~/.bashrc # file is restored
echo "export SHEEP=lambs" >> ~/.bashrc # Done correctly this time!
git commit -am "Added SHEEP to .bashrc"

Assuming you built your .bashrc with a bunch of similar echo commands and you just want to retrieve them:

Is there a location where a session's commands are stored?

If you run history, you should be able to retrieve the previous commands. For more information, see man history.

In your case, you might find the output of history | grep bashrc useful.