Is it possible to reopen a closed branch in Mercurial?

You can just hg update to the closed branch then do another hg commit and it will automatically reopen.

The closed flag is just used to filter out closed branches from hg branches and hg heads unless you use the --closed option - it doesn't prevent you from using the branches.

try with following:

hg pull && hg update branch_name

Now make a small change to one of the file and then commit it

 hg commit -m "minor change"

then push it

hg push -b . 

Now you should be able to work normally.

You can reopen a branch by using the "-f" flag when 'creating' the branch.

No, this command will create a new branch with the same name.

Just forget that it's closed. Switch to the branch, make the changes and commit. It will be automatically reopened. When you're done you can close it again.