The answer by NautArch is great and will surely help you mitigate this problem in future campaigns, but it doesn't help much where you're at currently. Here are a few things I've personally tried as a GM.
Make New Friends
And by friends I mean enemies. More specifically, an arch nemesis. Make one of the enemies the PC has defeated before come back for revenge. Make sure it's a personal revenge directed only at one of the PCs; if you make it an arch nemesis of the group it will become the arch nemesis of the involved PC.
PC: "Hey, I thought I killed you! I cut off your arm and stabbed you in the neck!"
NPC: "Yes, I died in pain. But some new friends took pity on me, filled up the hole in the neck, grafted a machanical arm on me and raised me back from the dead. You will now taste sweet revenge!"
Repeat a few times and make sure to constantly make the enemy stronger and more involved in the main plot. Soon you have a PC that's deeply involved.
Make Old Friends
And by friends I mean friends and family. Every PC has friends and family from when they were younger, even if it's not specifically stated in the backstory.
GM: "A letter is delivered to you. It says that your parents are held by a lich for ransom. With it comes a ring that you recognise as your mother's."
Naturally, the lich is involved in the main plot somehow and helping out the parents is a bit more involved than first anticipated. After they're rescued, it turns out the people you rescued are doppelgangers and that the real parents are either still prisoners or safe home in the village.
Make New Memories
Anything that happens or has happened during the campaign can serve as a hook for stuff to come back and haunt the PCs. If there is nothing in particular you can draw on right now, try to set it up.
GM: "Turns out, this is the home of one of the guards you killed in the town. She would have been here last night protecting her family, but thanks to you the monsters had no problem butchering the family."
Any situation that might serve as a plot hook in a backstory can be introduced during the course of an adventure instead.
Make Old Memories
Let an event remind a PC of something that happened in the past. This gives the player a chance to flesh out the PC and add things in a natural way. Also, repressed memories can work. It can be hard to pull off properly and it cannot be used too often but it's a powerful tool.
GM: "As you realise that you've been cheated by the merchant you recall something you'd rather forget. A time when you cheated a friend in a similar manner. Who was the friend, what did you do and why?"
PC: "I was fifteen and needed money for a new knife I wanted. So I made a bet..."
This is great for adding needed flavor or plot hooks to a character. You can even be pretty blunt about it.
GM: "A person from your past sends a letter to you that is deeply troubling. You need to help this person out. Who sent the letter and what does it detail?"
Make sure to ask follow-up questions to get a better grip on the situation.
Not only does this technique add hooks, but it also lets the players be a bigger part of the narrative and makes them more invested. It can really work wonders to make someone more engaged in the story, partly because they now get to play the exact story they want.
Just Make It So
Make the PCs be part of the narrative without any prior explanation. The explanation can either be made up on the spot or something the party has to investigate.
- When the Big Bad starts wielding an amulet, one of the PCs react to it. Why is that?
- Turns out, only the blood from one of the PCs families is adequate for a certain ritual. If the PC fights off the baddies, maybe the family of that PC is an easier target?
- One of the PCs inherit a small statuette. It seems to be magical, but no one they ask seems to know how. There is, however, a mage that specializes in magical statues and figurines. Time for a new quest?
Whatever you do to help out the situation though, NautArchs answer is still in effect. You need to feel your players out and try to understand what it is they want with the campaign. You can even have a Session Zero in the middle of the campaign if needed. This is where we are, but where do we go from here? This is what I tried to do, but what do you want to do?
Anyway, good luck GMing. It's tricky business sometimes and much to learn along the way.