To set things up so that you don't encounter this problem again, you must background all usages of xprompt in your .twmrc file. You probably have some lines in your .twmrc like
"Rlogin" !"rhost=`xprompt -re -p Host` ; test $rhost && xterm -s -title $rhost -e rlogin $rhost &"To background the command, you should put everything within the double quotes (except the & at the end) in parentheses, like
"Rlogin" !"(rhost=`xprompt -re -p Host` ; test $rhost && xterm -s -title $rhost -e rlogin $rhost) &"