If you’re carry-on is too heavy, the airline can charge you an excess baggage fee. This will vary with each airline. Furthermore, some airlines do not enforce or weigh carry-on bags. On the other hand, some airlines do not have a weight restriction on carry-on baggage, but you should be able to safely stow your carry-on in the aircraft overhead bins without assistance: don’t bring carry-on luggage that is too heavy for you.