"Operational capacity" is worded kind of oddly, it refers to the amount of resources you can collect through missions throughout the day. It's auto-generated daily so you can get more resources if you work your horses.

Workshop usage is dependent on prestige, and if it drops too low you lose the ability to use a workshop until you get it back up.

workshops is dependent on prestige:
0-20%-1 workshop
21-40-2
41-60-3
61-80-4
81-100-5

I just looked at your ec. to get it back to 40+ shouldn't be hard or expensive. here's a few things you can do tonight to get it up over 40 so you can start using the 3rd workshop again
1. comfort-add some saddles or bridles. you only have 4 boxes so you only need 4 to offer them. but i'd have more. 3* would give biggest jump but those are most expensive. any of them will give you a bump. you are so close to 40% you don't need big jumps now to get to 40+
2. meadows-make sure you have good quality meadows for pasture. you only need 4 acres so will only need 1 small pasture
3. competitions-change all of your comps away from "less than 20 wins" and "low level horses"
4. you can add more boxes to help with capacity. but that's going to need more tack and pastures. if you go this route bigger boxes will help with comfort more.

Doing any 1 or 2 of these things tonight will get you up over 40% at the update. after that you can work on getting it higher. That will take time and money. the few things i listed above are quick and free or not that fast.