If you changed the date and time settings on your 3DS all time-based events that happen on the 3DS system will be stopped for 48 hours, and then it will start up the time-based events again.
The problem should be that you changed the date and time settings on your 3DS - if you did this, just wait 48 hours and Hotel Richissime should let you work there again! :D
If you didn't change the date and time settings, or you have already waited more than 48 hours, I recommend contacting Nintendo. Just click here to find your region, and find your way around the site to the 'Contact' page. :D
Hope I helped. :)
Source and Experience
