That's why (if this situation was even remotly realistic, because let's be honest, ti's not xD) We'd move underground where the bombs can't reach us. We'd construct large Anti ship battery's to shoot back. sure they can bomb us and it won't look good for us. But controlling space isn't a garantee to win. In the case where we had more planets, we'd just call in reinforcements and then it's a matter of keeping the invaders from controlling the ground until back-up can arrive.
Building underground shelters with room for 7 billion people is such an enormous effort, that we could never even begin. The massive amount of construction needed would be more than everything humanity has ever built, in its entire history, combined. I don't even want to speculate how expensive that would be. All that, just so in case of an attack everyone would be safe. So building something like that is for all intents and purposes impossible.
Assuming against all odds, we had such an extensive underground shelter system, the logistics of getting even a fraction of the total human population into them is a nightmare. Evacuating just a single city is difficult enough and usually takes weeks or months. And that's with help from outside. In many cases you can't build shelters close to where the people are, you would have to move them there, which takes time that you don't have. So even if you had the shelters getting everyone into them would be impossible with the time you'd have.
Should you somehow solve that problem, you run straight into the next one. How are you going to feed 7 billion people? Even assuming you had vast stockpiles of non-perishable food, building up those stockpiles would take much longer than consuming them. And even if you had a stockpile that could last years, it would eventually run out because you can't grow crops underground, at least not in any appreciable quantities.
I could go on, but I think I've made my point. Taking shelter is simply not an option.
Next is shooting back. None of our military can fight anything in space effectively. Only if the invaders actually tried to land troops would we have any chance of attacking them. Air forces would be the first line of defence. Let's assume they don't have a too big technological advantage over us, so they won't have shields or any kind of weapon system we couldn't build ourselves. Additionally they won't expect us to fight back, just to give us an advantage. So the first wave of landing craft will get taken out by our fighter jets, what remains of their first wave pulls back to their troop ships. Now they got a bloody nose they will analyse the situation, note where the fighters came from and bomb those pesky airbases to rubble. They will also attack and destroy every aircraft carrier and the ships close to them. Next they will launch a decoy invasion with the intent of drawing the remaining fighter jets out, if none are left the decoy invasion will proceed as normal, and at least some of them will get destroyed by AA guns and ground troops. The response will be a quick attack from orbit to wipe those installations and troops out. But some troops will make it to the ground, though to avoid too many losses, most of the transports are empty and will return if they don't encounter resistance to pick up troops. Now they can land in force, likely targets are vast areas with low population density, to consolidate forces far from any resistance. Once they are on the ground, they will be incredibly hard to defeat. At that point, launching nuclear missiles would be the only option to get rid of them. But that requires us to know where they are, which is difficult because they will definitely have destroyed all satellites, and air surveillance is not an option either. But at least some of the forces could be located and destroyed by land based nuclear missiles (submarines need satellites to communicate over long distances), provided they don't get shot down before they can detonated. The idea of doing that isn't new, and we do have the capability to build space based weapons that could intercept ballistic missiles (cruise missiles don't work due to lack of GPS). It will get progressively worse from there. Any time they encounter resistance, they can call in an orbital strike. All we can do is delay the inevitable at that point and the longer the fighting goes on, the more the civilians will suffer. How long do you think it will take before governments start to capitulate either because they see the futility of resisting or because a faction with that view will take over and surrender?
And those fancy anti-ship batteries, even if we could build them, each would only be able to fire once, before getting destroyed by an orbital strike.
You'll note, that bombardment was kept to the bare minimum. No strikes at civilian targets, no attack on industry or infrastructure, airports excluded. Only militarily relevant targets get attacked. Just how bad would it get if the invaders were more aggressive and pragmatic.
TL;DR
Building underground shelters for 7 billion people is impossible.
If you have them anyway, getting the people into the shelters takes too long.
If you get them in anyway, they will run out of food.
Fighting an opponent that controls orbit is delaying them at best, the side with spaceships will win.
