When you see Process Completed in the Terminal, that Shell session has exited, you cannot type anything further, and you would need to quit and relaunch the Terminal. If you did not enter the word exit, or type a control+d, either of which would end the current Terminal session, then you have an entry in one or more of your Shell dot files that is exit and killing your Shell. You would need to fix that.
If you do not have a programmer's editor installed already, then Install the free BBEdit text editor if you haven't already. It is a trial that when expired, becomes a free tool.
Quit the Terminal app. Launch a Finder Window and select your home directory on the Favorites listing. Type Shift+cmd+N to create a new folder and name it DOTFILES. Now, press shift+cmd+. to reveal the dimmed hidden files in your home directory. Your Shell's dotfiles will be there.
Bash
.profile
.bashrc
.bash_login
.bash_profile
.bash_logout
Zsh
.zlogin
.zprofile
.zshrc
.zshenv
For the respective Shell that you are running, locate and move (drag) the respective dotfile into the DOTFILES folder. Once in the DOTFILES folder, right-click on each dotfile and choose Open With BBEdit. Search in the newly opened file for the word exit. If you find one, place a comment character (#) before it. Then save the .dotfile, exit BBEdit, and then move that dotfile back into your home directory. Note, that any application run in a .dotfile that sends an exit command to the Shell will also kill the Shell to watch for those too.
When done, in the Finder window, click shift+cmd+. to undo the previous show hidden files command. Quit the Finder Window.
Now launch the Terminal application. It should not produce the Process Completed message.