I'm sorry to hear you had a bad experience- I know how frustrating it can be to have unexpected loss, particularly here, where it feels like it could have been prevented.
I'm not associated with Linode; I don't even host anything there, but I can see things from their perspective on this.
With a service like this, you'll sometimes see people who never cancel a service, they just stop paying for it. Maybe their card expires and they decide not to renew, maybe their business closed, or any number of other things.
If I were in their position, I'd certainly reach out to you- I'd warn you it was ending soon, and I'd remind you to renew. For one, it'd be the decent thing to do, and for another I'd want to get the renewel revenue! As the expression goes, 'It's easier to keep a customer than gain a new one.'
That said, if the time came and you hadn't renewed, I'd have a decision to make.
I want to keep you as a customer, but from my perspective, I have no indication you want to stay as a client- You haven't sent me any email about it, and you're not paying me anymore.
I'd probably archive off the data for a while, and shut it down.
My mental thought process would be akin to "I'll turn it off, and keep the files around for a few weeks. If he's using the site, then he'll notice when it gets shut down, and pay to stay with us. If not, Good luck in the future!"
From my perspective, it looks like that's exactly what Linode did. They gave you 20 days; That's nearly 3 weeks of the server being off, without you even writing them an email to say "Hey, Noticed it's off and I'm not paying you, could you please give me a few more weeks?"
I understand it's very frustrating for you, but I really can't blame Linode at all.
The other thing I'd touch on briefly is that you really should have backups.
I don't want to harp on it, because I know it must feel like kicking you when you're down, but servers fail..
What if Linode's datacenter caught fire? What if they were bought out by a company who wanted to convert them all to Windows servers? ;) Who knows. Things happen. Having the service turned off for nonpayment is among the smallest of problems.
If you were running a nightly backup job at this point, you'd be very frustrated that you had to copy everything back, but you'd have not lost anything.
In any event, I'm sorry that things didn't work out, and hope that your experiences with Linode (Or whomever you use to replace them) are more positve going forward.