The likely reason for there not being food is underproduction. Causes of that could be not having enabled planting for every season, planting textile crops or crops that need to be processed before they are edible (if the textile crop fields are too large it might be fine on embark when you have only 5 seeds, but end up using up all planting time once the seed count is high), lacking seeds, perhaps from cooking the plants directly (cooking does not give back seeds), or being bottlenecked on field size or planter count. Food will rot if not in a food stockpile, but the rate is pretty slow and easy to outproduce, also if underground the rot will produce miasma making it easy to notice. The next likely reason is that a single dwarf might have become unable to path to food. In such a case the dwarf is likely walled off from the fortress in either the caverns or the surface, because anywhere else there wouldn't be water and they would die of dehydration first. Disabling web gathering at game start helps prevent dwarves being stranded in the caverns. It's also possible all food was forbidden or there was an active civilian alert.

It's a bit complex in that there are many ways to fail, but again the winning formula is simple. For me the learning curve was first giving up because I didn't even know what I was looking at or that z-levels existed, and then a year or two later watching a video tutorial that quickly laid down all the basics, and finding the game relaxing to play after that. Unfortunately I don't remember what the tutorial was and some tutorials out there are pretty mediocre, e.g. anything that recommends making a refuse stockpile outside is not adequately conveying that sealing off your fortress from the outside world is key, and IMO building an indoors well should be mentioned as something you need to do even if the process's not covered.