Could you give me some instructions as to how to proceed?
You need the Cisco console cable (pale blue color) or equivalent, a computer with a serial port, and a terminal emulation program. The console cable goes in the other RJ45 on the back of the AP (labeled "Console" in pale blue, normally). Start your terminal program and set it to 9600 baud. Turn off the AP and turn it back on - you should see a whole stream of console messages. One of those messages will be the version of the software the AP is running, for example:
Code:
Cisco IOS Software, C1250 Software (C1250-K9W7-M), Version 15.2(2)JA1, RELEASE SOFTWARE (fc1)
K9W7 is Autonomous, K9W8 is Lightweight.
If you press the [Enter] key on your computer, you should get either a command line prompt or a password prompt. The default username and password is "Cisco" (without the quotes, and with an uppercase C).
Much Cisco documentation is available without a login. For example,
here is the Configuration Guide for the 1250 in Autonomous mode.
Normally an AP in Lightweight mode will register with the controller and be configured from the controller. If the AP does not successfully register with the controller, you might need to manually clear its config with the procedure shown
here.
If it AP is in Lightweight mode and being obstinate, you can use the procedure I posted
earlier to enable many commands normally disabled in Lightweight mode.