Terminal prompt messed up after changing value of PS1

$PS1 is an environment variable that tells yours shell how to format the prompt. Changing the value with export, as you did, only applies to that session; so you can just open up a new shell (not a sub-shell) and see a familiar prompt.

Otherwise, if you really want your prompt back in this session itself, just rerun your shell's preferences file. For Bash users, that is:

. ~/.bashrc

I don't know if that will work (at least, without causing other issues) for Byobu; but you can just set the prompt to a sane value like this:

export PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '

Or, if you don't want to or can't have a colour prompt, a simpler one would be:

export PS1='\u@\h:\w\$ '

where \u is your username, \h is your hostname, and \w is the full (logical) path of your current working directory.

You can set PS1 to a reasonable value for this Terminal session using:

export PS1=$'\h:\w\$'

The old value should also be restored after closing and restarting Terminal, a reboot, etc.