When all else fails, random pages of punctuation work for me. Or watch a Monty Python video, look at art, hit the pppppppppppppppppppppppppppppppp key several hundred times.
Count your blessings, drink hot tea, draw, play your guitar, fidget, pray, meditate, look at online videos of Israeli special forces fighting. Refocus, breathe, smile....read some poetry...
Take a walk, dont forget to breathe...connect with something from the past...or someone, even if its in your mind...think of something that has really impacted your life. That has made it worth living, that reminds you that there is a sun that will rise tomorrow (whether you like it or not), someone who reminds you love is real and not just some imaginary illusion.
But don't hyperventilate.