It is too much asking if you tell us what change you did?
Asides that
If you got access to one of those APS for example the AP 124 which has a console port check what config it has and if its correct
You can always console the AP boot it up and check the config with on the apboot
When ap is booting you press enter enter till you get to the boot
do this command
print
It will show you who is the master, servver ip, check if its correct(or you can show us the output)
you can also do dhcp command to see if he is getting ip(if you are getting from the DHCP server)
If you put it statically then the print command is enough
IF you see everything is correct
try booting the AP and look at the output and look if it has an error or the reason of why is not contacting controller, with it should tell you.
When i get this kind of situation in which the AP cannot contact the controller i always go and console the AP and most of the times the problem is soemthing in there...
If you see that in the print output something is incorrect then you can correct iit via console...on the cli check that out... that is what i would do, and well thats is what i do when i get this kind of issue...
Actually i dont see a way to troubleshoot it asides from consoling the AP, as the AP cannot contact the controller you have to see whats up with the AP :) and you do that consoling it...
.