How to make a comeback?

Attack as a team. Focus on one DG to get a kill if you can. Capture flag, lock the flag with the flag locking device (250g in shop) go to the next flag. You need to do this soon, before you go way behind in warscore.

Obviously it's going to be hard, but here's a few things that I've noticed helping me when that happens.

1 - Engage creep waves closer to your towers and further away from the enemy DG's (usually this can be achieved by not following your own wave to the front line and letting them die further out)

2 - Be as mobile as you can (go all out on move speed upgrades) and try to hold down as many flags as you can, this either forces the enemy to split up, or starts upgrading your war speed and buffs, which makes a big difference.

3 - Try to work with your team mates to lure single enemy DG's after one of you and then have the pack fall upon them out of the fog of war.  Even a DG that's 4 levels above you won't be able to take on 3 of you at once.  

If you can get one kill that way it puts them on the back foot, you should be able to either get another kill on the outnumbered remaining DG's or you should be able to regain good map control, which will improve your buffs and income

Small map-Forget flag capping. They can have all the flag, but they will have to deal with you and your tower, with you camping at the tower base. Only cap flag if you score a kill, then back to camping.

Big map-Flag capping spree. Avoid confrontation. Have your team ambush lone target trying to cap flag (you can often find opponent team doing that, due to your team's capping spree, and they need to catch up on flag capping which they thought they can split up to do it faster)

Also, don't buy new creeps (priest, angel, etc). Last thing you want is giving opponent more exp+gold.

I've come back on games that the enemy got 22 kills, and my entire team only got 6(all 6 by me). Just keep the flags capped, build up your warscore, get giants, capture enemy portals, game over.

turtle more or less , focus on getting 1-2 demigods really powerful vs demigods (for ex regulus/ub going atk items) and the other one focusing on you surviving .

when you get a kill use it to your favor by capping flags together and pushing back creepwaves ,

another good thing is to learn which items give the most damage/cost , the gloves for 400 for 25 damage is very good as is lightning thingie for 1500 and warlords punisher for 2250 . small cost to make your dps do a lot more damage , and the lightning (whatever its called) also helps clearing creepwaves and adds mana enough for another nuke.

no.. turtling will make you lose.  You have to use flag locks to beat them in warscore if you can't get kills.

If they have more warscore and more gold, you have lost.

Or you can try backdooring.

I kind of hate to give up my secrets, but I can say kills don't matter unless they have 4 times more, or more.  If they only have double, you're fine.   Actually, lots of the times I'm happy when the team is leading kills, because I know they're starting to think they've gotten the game.. then a few minutes later they ragequit.

as you turtle you deny them in creeps and get more gold yourself , upgrade gold inc at lvl 1 as soon as possible and keep farming theyre creeps, ordinary creeps cant push back your towers when you defend and if they get priests you get a big gold/exp gain .

warscore doesnt help them that much if they cant afford the upgrades. use the advantages they give you to the fullest more or less

You can afford the nessisary upgrades with the gold you get with hardly having any creep kills or even demigod kills.

well the only thing that easely breaches a turtle defense is catapults/giants , and on a small map that takes long time to get and on a large map you can tele around and snipe flags , i dont know what you are implying for as necessary upgrades , but mob health/mob attack should be the best for going on the offensive as priest give way to much of an exp/gold boost .

what upgrades do you think of then ? were talking about 5 mins into the game here so ?

Backdooring is one of the few effective strategies. It is extremly difficult regardless to make comebacks.

Backdooring = Get the enemy spawn portals even if you are running through towers to get to them, lock them down, port out.

I've had many comebacks since release, and most of them seem to come because my opponents made the mistake of continuing their gameplay style while pushing up to my citadel. For example, on crucible there is a high concentration of towers around the citdadel, so any enemy push is going to be short, but if you in turn ping one of their players and gain the upper hand, they have a very long way to run before they get to their towers. You can easily wipe thier entire team in 20 seconds or so after the first one falls in this manner as it will be 3 on 2, with their 2 sitting between the player who pushed out and chased one down, and the other two back in the base, and from there the entire map is at your mercy for ~20 seconds.

I think the same is true on most maps, but I guess unless you have a speed advantage you can't use this very well. Buy the Wand of Speed or something.

Obviously this is never going to work against good players, or if the level advantage has gotten out of hand.