A Walk in the Park
When you are stuck at something, it's probably time to take some time off. Even if there is an imminent deadline and you HAVE TO keep on working, it will probably be counter-productive.
What I've found to work best is taking a walk in a park. If you live in the UK, then there's probably a park in walking distance from where you work or live. When you are stuck and can't get the juices flowing, hit pause and go for a walk. If you have a dog take him with you.
Like I described last week, when at the park be mindful. Don't keep thinking about what's waiting back at home or at work. Enjoy the fresh air, the scenery, the birds and the trees. Take it all in and let go of what's troubling you. After about 15-20 minutes that your head will have cleared, head back.
Once you are back, prepare a fresh drink (I always go for coffee) and dive right into work. Most probably you will find that you will have a fresh perspective and new ideas to bring in.
To add a bonus to the above, if you are working on a team project and you are all stuck, try getting the team out of the office and to the park. Spend 20-30 minutes playing a game or sitting and talking about irrelevant stuff. It will boost energy and get you ready for the stuff waiting to be tackled.