I sent a letter from London to Darlington about ten days ago?

its for student finance, i sent it well over 10 days ago and it was normal class. should i be worried? i just checked the website and it says it hasn't arrived yet. what do i do??? it had really important evidence for my application and im freaking out!!! what do i do? should i contact the post office? but which one? idk, does anyone have any suggestions please??