Here's a question for you, does it matter if it's your fault or not?
If you answered yes we cannot help you. If you answered no then you have to ask one more question, was it your allies fault or the enemies fault?
The latter question does actually matter, if your allies are simply feeding they will be able to come back late game, so your goal should be to help defend objectives instead of push enemy objectives down. Get your allies to farm safely on their side of the map and get your own items as fast as you can, but make sure they are ready to defend/take dragon.
If the enemy is outplaying them you have a very different problem. Your allies might have had a chance before the first kill happened, but their opponents are snowballing. Help your allies push objectives down as fast as you can and rush the team fight phase. Once your team is grouping continue farming and holding lanes that your team are not actively pushing, and make sure your team does not engage while you are not near. You want your team to siege so remind them to have wards around so they do not get flanked.
The enemy team will likely have a split pusher who has to be stopped, so be ready to run to defend your inhibitor towers. Unless the team fights are going well your 0-5 ally should not be trusted in handling the split pusher, it's your duty now.