Don’t know whats causing the issue? Your developer must have done something wrong for which you’re unable to access the dashboard. Anyway to investigate this we need your hosting access along with WordPress dashboard Administrator access so that we can have a look at what’s going on with your site. Why we need hosting access? Well we need to debug the code for which we need hosting access. You can send us the access in private comment that’ll be all so that no one accepts us can see this. Check the screenshot to know how you can post a private comment here: http://i.imgur.com/1oUlQNl.png
Let us know.