I thought it might be fun / therapeutic to share your failed fortress stories with brief timeline. I might be able to fill this thread to it's limit just by myself.

The Tragedy of the Dwarves of Lalturbomrek
Lalturbomrek started on Osmlo Xagu in Spring 551 inside a nice hill. Farming, brewing, crafting, simple meeting hall & dorm all up and running. Good wood stockpile and rock blocks were rolling out. One migration wave made the population about 14 or 15. Then in summer 552, while absentmindedly channel out a moat in my three wide entry tunnel, disaster struck. The miners didn't realize (or check) what they were above the meeting hall. When they hit water, they dug again and it was all over. The meeting hall and all levels below filled with water. In an attempt to drain the fortress, both miners were caught in shallow water until winter hit and the mouth of their emergency tunnel froze shut trapping both to their death.
Since the only two picks the fortress had were gone, our dwarves had to try to rebuild without digging. Stockpiles were shrunk & moved. Wood was cut and stacked, walls & floors were placed to redirect the flooding and protect the clumsy. A new temporary dorm was setup. 3 dwarves were stuck in the crafting area while attention was directed elsewhere and perished due to dehydration but made stone crafts until their final minutes.
Burials, slabs and tons of booze kept spirits surprisingly high.
Still the migrants came. So did the elves. For almost a year and a half there was no pick to be had and no digging done. Finally in spring of 554 a migrant wave came with a pick (at least that is what I think, I'm not sure really). Digging of some long delayed paths to allow wagons happened opening up the entrance to fortress.
Dwarves came will all sorts of good stuffs. New areas for stockpiles were created and slowly (very slowly) the 50 adult dwarves started to move everything around and dig deeper and down further into the ground. Due to all the inactivity for the two prior years, at this time probably 20 babies were born as well.
Happiness was through the roof. Parties were happening, the fortress was on autopilot.
But these setbacks took our dwarves minds off of defense. Some thieves had tried to steal, but dogs and some crossbows took care of the easily. Nobody thought it was time to get a good seal on the only entrance.
Then the minatuar came when nobody was paying attention until to late. He made his way to the grand hall and just started killing. Other dwarves went mad due to the losses.
It is now rapidly approaching winter 554. Nobody is happy. Almost everybody is raging. Nothing is getting done.
The sane dwarves are thinking about gathering outside, sealing up the front entrance and abandoning the fort to it's quick demise. Perhaps in 555 somebody can unseal, go back in and reclaim the wealth to start another fortress a few miles away on the other side of the hill. But nobody in their right mind will want to actually live in Lalturbomrek again.