Make sure that the carpentry workshop hasn't been suspended. Put the cursor over the building using the "t" menu. If there's red text saying "Construction Suspended", the building will never get built until you manually unsuspend it. Construction of buildings can get suspended for a couple of reasons, here's two common ones:
A) Dwarf A picks up the stone (or block, or logs, or whatever) you designated to build the workshop. On his way there, a raccoon runs by him and scares him. He drops the stone and construction is suspended.
B) You designated a building material that for whatever reason is inaccessible to your dwarves (across the chasm and no bridge, inside a locked door, item destroyed). In this case, as soon as the dwarf checks for the material and cannot find or reach it he will suspend construction on the workshop.
Remember, once a building gets suspended it will never be completed unless you manually remove its "suspended" status under the 't' menu.
As for your bridge - the same discussion about being suspended applies. Check your second bridge.
Also remember for bridges that both ends of the bridge must be fully cleared of stones, ore, etc. to be successfully built. The architecht does this automatically, however he must be able to reach both ends of the bridge to do so. If he cannot access both ends and needs to, guess what - the bridge is suspended.
Finally - your food situation.
The "other" category in your food menu is deceptive. All sorts of stuff falls under "other" - seeds, spawn, tallow, prepared meals. Not all of the food in "other" is directly edible. Tallow, for example. So it counts toward your food score but your dwarves cannot eat it.
If you have 1200 other but your dwarves are starving, build a few kitchens. Use the "kitchen" menu to see what inedible items you have a whole lot of and make sure they are set to allow cooking.
You can make prepared meals out of two inedible items - plump helmet spawn(inedible) + tallow(inedible) = easy prepared meal (edible)
So this might be a way to salvage your fortress that's out of food