Others have given great explainations in broad strokes, I’ll go into nitty gritty detail because this stuff can get really confusing and the game doesn’t teach you about it, at all.
Every building piece you place has a decay timer. Connected building pieces shared a decay timer. For example, if you place a foundation, then snap another to it, then snap a wall on one side, all 3 pieces will share the same decay timer.
The decay timer starts at a certain value and it ticks down, second by second, until it reaches 0. Once it hits 0, the building enters the “abandoned state”. While it’s in that state, anyone can demolish the whole building using the radial menu, even if they aren’t in your clan. The abandoned state lasts for 24 hours and after that your building goes poof.
That’s the “bad news”. The first bit of good news is that you can “refresh” that timer to its initial value just by being within the rendering distance of those pieces. You can see the value of the timer by holding down tab while facing the building piece and being close enough to it. You can also see it by having a repair hammer equipped.
The second bit of good news is that the value of fully refreshed decay timer depends on how many connected building pieces your building has, and also on their tier. The more pieces, the higher the timer. The higher the tier of those pieces, the higher the timer.
Every placeable – e.g. crafting stations, chests, wheels of pain, torches – also has a decay timer. The value of the decay timer is taken from the building where you placed it. If you didn’t place it in a building (or on the ground very close to a building), then the default decay timer it has will be very short, along the lines of 30 minutes or 1 hour, depending on which placeable it is and on the server settings.
I already explained how to prevent the decay, but there’s more to talk about in order to understand the message that popped up.
So, remember how I said that the “fully refreshed” decay timer depends on how big your build is? There’s another detail I didn’t mention there: that maximum decay timer is capped, per server settings. The game calculates the decay timer for your building based on its size and tier, and if that value is bigger than the server decay timer cap, it brings it down to the capped value.
Normally, official servers are configured to cap the decay timer at 168 hours (7 days). So if your build is big and in T3 and the game calculates that it should take 250 hours for it to decay, you won’t get 250 hours, but 168.
During the summer holidays, Funcom sets the decay timer cap on the official servers to 336 hours (14 days). Here’s something most people don’t understand, which can lead to building loss: this doesn’t slow down the decay or even change the current value of your decay timers.* This means that, if you had 100 hours on your timer before they changed the max decay to 336, you will still have 100 and not 200. Also, if your building was capped at 168 hours before the change, it does not mean that you will automatically get 336 after the change. Remember, you might have had 168 because the game calculated 250 and then brought it down to 168. If the cap is raised to 336, but your max timer is 250, then you get 250, not 336.
This can be super confusing for people, but it’s really important to get it right.
I already explained the decay process above, but I just wanted to mention that if your building is in “abandoned” state and nobody has yet demolished it, you can still refresh it by being there.
If your building was demolished while abandoned or if it fully decayed after 24 hours of abandonment, then it’s gone and you can’t get it back.
There are three more details that are useful to understand. One is how demolishing an abandoned building works. When a building is in abandoned state, you can long-press E (or whatever binding you use to interact) in front of a building piece or a placeable, and select “demolish”. It has the same icon as “dismantle”, but “dismantle” requires you to be the owner.
If you demolish a placeable, then just that placeable is destroyed. If it has an inventory, it’s dropped as a loot bag. If there was another placeable attached to it, then that placeable will lose stability and also be destroyed.
If you demolish a building piece, then all the connected building pieces will be demolished, too. Anything that derived stability from them will also be destroyed.
The second important detail to understand is what happens when the building fully decays without being demolished. The server does not run the simulation for the parts of the map where there are no players around. That means that a building can be fully decayed for days, even, without being actually destroyed and the server only destroys it when someone stumbles upon it. That’s why it’s sometimes possible to stumble upon a building and watch it crumble down before your eyes – it’s not that you were supremely lucky and just happened to stumble upon it at the very instant it decayed 
The third important detail to understand is that there’s a bug with the decay timer and abandoned state. If there are players around the building when its normal decay timer reaches 0, the building might not go into abandoned state, even though the decay timer is stuck at 0. If that happens, everyone needs to go out of the rendering range (or log out), so that the game will update the abandoned state properly.
I hope I didn’t bore everyone to death with this, but it’s stuff that I learned by playing, sometimes in a painful and frustrating way 