If DHCP is not found, AP reverts to defaul behaviour, which is to set its IP address to 10.0.0.1/24. You have a 5 mn window to access it... just set your IP to any address in that range (or let the Ap internal dhcp provide you with one), and connect to it...
You can also reset the AP (with Mode switch , waiting red light), and then connect to http://10.0.0.1 using password Cisco...
hth
J