The store is back up and running - apologies for the problems here.
The cause was we were using Yahoo's open API's to retrieve currency exchange rates. Lots of websites use these free services and have done for many years. Without warning, Yahoo has disabled access to these API's.
On Construct.net, this caused a silent failure. On Scirra.com unfortunately if any currency other than GBP was selected it was cause an exception.
We've moved to a paid forex service which should provide better stability and reliability now - and don't anticipate any more problems here going forwards.
Again, apologies for any concern or inconveniences caused and thank you everyone who took the time to report this.