When this message is displayed on an event hotel on the Front-End, one of two things can be happening.

The first case could be that the "Accept bookings" toggle has simply been unchecked, as shown in the image below. Checking this box again, if the event hotel isn't affected by a cut-off date, will re-enable it for guests to book into.

The second case is if the event hotel is affected by cut-off, either at the event level or the event hotel level. 

If it's past cut-off for the event, but you still want this hotel to accept bookings, you can click the second checkbox, as shown in the red outline below, and set up an event hotel level cut-off date. Doing so will enable the event hotel to continue receiving bookings even though the other hotels unless also set up this way, can no longer be booked into.