If you want to reinstall, backup your /home/username folder to a usb stick or cloud.
Start Manjaro and search in menu "man" and press menu-item manjaro-live
Provide your password and hit ok
Select your Manjaro taste and hit ok:
The latest version will be downloaded and after that the computer will reboot in the manjaro version live.
After reboot, you will have a live version of Manjaro loaded in ram. You can test it or install a fresh manjaro . The installer will start once you hit the manjaro-install desktop shortcut.(warning!: if you reinstall manjaro, all data will be erased)