Could be a problem with your internet connection or simply the server acting up zlobby.
Try contacting the support for the game by clicking the tab on the right side as ArmorGames can't help you with this problem because the game is run from servers off-site by the game developer... ok?
If sometimes try to reload current page, maybe even a few times.
In any case In the last month there were a lot of adjustments on this game, and it is not possible to enter the game during the maintenance. Sometimes the server is too busy or overloaded.
I agree on the server issue here... sometimes the server is down due to maintenance or even without proper notification/explanation, sometimes even in the middle of a mission. But there's nothing we can do about that, other than wait it out and refresh. I did see some similar mentionings in the TLS: DZ Support Section and the TLS: DZ Forum, have you tried checking there? =)