take a break, go to the park. Just get away from it. Or ask an expert.

yep... a break especially if you are tired

getting someone else to look at it can help even if this other person is not an expert

walk away from the problem. I had an argument with a project manager once... couldn't find the solution for several hours

The team walked out at 10pm ... Came in the morning and solved it in 20 mins WTF !

Had the project manager listened to us he would have saved the company thousands