@FineandDandie , I'm so sorry to hear this. Almost certainly the reason the buyer gave is not the truth, or they would have tried to negotiate. Not that it makes a difference to where you are now.
Once the shock has worn off, I hope solutions come to mind so that you can get things moving again. Off the top of my head...
Could you decamp to an AirBnB in the area where you need to be, and meanwhile get most of the furniture back into the flat so that it is staged for re-marketing?
Were any other potential buyers interested in the flat, and could the EA contact them to see if they are still in the market?
You don't mention your onward purchase, but it may be that your seller is prepared to wait.
If it's any consolation, I lost two buyers before completing on my sale last week, and ended up getting a bit more for my house than I was expecting.