Note the date, time, and Trade ID number - you can see this in the trade software, f.i. FXCM trading station - and contact your broker about why you could not close the position. As to my knowledge, USD/JPY can normally be traded 24 hours, but maybe there was some market or server glitch at 19:00. Your broker should be able to answer the question.

A possible reason is that you use an US demo account and are not permitted to close any position, but must open a position in opposite direction instead. But then this should not affect only USD/JPY but all your trades. If you have an US account, please let me know - those accounts have some special restrictions that we'd like to test. All our accounts are from Europe.