Wow, this can be tough. I think the important thing, when one is "very depressed," is to reach out and get as much help as possible. I recommend working with a pro (therapist or physician) and talking often with listeners at 7 Cups (of course!) and reaching out to reliable, supportive friends and family.
At work, too, it's good to reach out for help--and see if others can help with projects or deadlines. When possible, it might be good to take time off or try for a lighter schedule. (However, it can be tricky to disclose mental-health conditions to certain employers, whereas others are wonderful about it and have Employee Assistance Plans to help employees get through tough times. Some states and countries have laws that provide protection, and some do not.) As for tips for getting through the day, I could say things like, Try to get some sleep and eat healthy food and definitely get some exercise to keep the fog at bay -- but when a person is very depressed, this can feel terribly hard to do! So, when depression gets very bad, I think it's time to consider seeing a physician and using appropriate sleep aides and other therapies and medicines that can give relief. And ... yes, depression can make it awfully hard to reach out for help, but it's crucial! Please don't try to "tough it out" alone. There are people like the listeners at 7 Cups who are waiting to help.