Ubuntu 24.04 doesn't detect my second monitor

Run xrandr in a terminal (on Xorg). If the second output says connected, the system sees it and you can turn it on from there or Displays. If it says disconnected, look at the cable before drivers. HDMI to DisplayPort adapters are the usual trap, passive ones only work one direction.

is it an nvidia card? that's the usual culprit. the open nouveau driver often won't drive a second output properly. open Software & Updates > Additional Drivers, pick the proprietary nvidia one that says tested, reboot. if it's intel/amd, or the driver is already right, try logging in with 'Ubuntu on Xorg' instead of Wayland (gear icon on the login screen, bottom right) and see if the monitor shows up in Settings > Displays then. and if it's a usb-c hub or dock, check the port actually does display output, a lot of cheap laptop usb-c ports only do data.