Networking 101. Check to see if the AP has power. Reset the AP. Reconfigure the AP.
If the AP is in working order then this should be the fix since you don't have an IP for this AP.
Maybe I missed something, but you said you couldn't ping it or get to the configuration page of the AP. What IP were you using if the IP you have on the map isn't an active IP for this AP?
Time for a network audit to make sure all your equipment is in place and active with all the right IPs and MACs listed.
