What things can you do to come back a Dota 2 game?

You are losing because they have better teamfight than you do

You must invest in the magical powers of split-push. Send your team to push all the lanes at once, and this will force them to split apart. If you keep doing this for like 20 - 30 mins, you will probably win if you do it correctly. Just remember, it doesn't matter if you are 20 kills behind and hopelessly out-itemed. What wins the game is structural damage, and if you're the ones attacking the ancient despite being 50,000 gold behind, you are the ones winning.

You are losing because someone snowballed

Play hyper defensively. Sit behind your tier 3's and scrape as much farm as you can. No matter how fat a snowballed person is, he probably will die if you all go on him at once. Make sure you use your abilities to their fullest, don't waste any ability. It's going to be hard, but it could be possible

You are losing because they are trying to split push

Play defensively. If they come knocking, send someone to answer. Getting a kill on them is good, but most of all just force them back before they do major damage on your towers. If you keep wasting their time like this, and get good farm during that time, then simply just hard push a lane, preferably mid since it's the shortest lane but other lanes could work. They are going to have to reply once you hit their tier 3's, and since you're ahead in terms of gold and xp you can destroy them in fights.

That's all the general ways you could lose. Probably the most universal in all comebacks is to make sure your team is working together. Dota 2 is a team game, and if you keep your teams morale up even in the darkest hour, you can easily (I say easily, it's going to be hard) make a comeback. If your team is busy flaming each other and being dicks and calling "gg", then your chances of making a comeback are incredibly low.

Depends on the situation and team combat

Sometimes you can hold off the enemy long enough for your carry to farm, sometimes a good engagement (usually has to do with bad enemy positioning) can turn the tides. If your team is getting picked off by roaming gankers, put wards in your own jungle, bait them into ganking (which is easy to do because they are getting cocky), plant sentry wards near the bait if the gankers are invisible, then kill the gankers. You can do this a couple of times to try to come back.