1. Do you get sounds from the Headphones out?
2. Do you see the Input and Output LEDs responding to your strumming?
(if you don't, take your guitar chord out of the Headphones out and plug it into the Input socket :p )
3. What happens if you connect your (active) monitor to the Main out?
4. Have you tried a different cable to the cab?
Let us know 
Mhhh...
Do you have any pedal or MIDI device connected?
Apart from this, I am afraid I don't know what else to think of, if not of a hardware fault... Let's hope someone more awake than me comes out with some obvious thing we're not thinking about... 
I just can't get it to work..
Did you check the output window. Mostly there is the reason for issues.
I once was seeking for a mailfunction for 1 hour .
The reason was that the volume was set to - infinitive. ( Can not explain how this happended.)
Yes, I have the monitor output set to 0db...
Can you make a picture of your Output screen - At least Page 1 and 2 of 6
Try tho following: unlik the monitor out in the checkbox. Turn main volume to the left and also monitor out.
set the checkbox monitor out link again and turn the mastervolume a bit. Both leds have to come up. Than set both volumes again to zero ( turn both wheels again completely to the left) and than turn the master volume wheel again ti higher the volume.
I have reported an issue to the support that sometimes the outputvolumes are different even when both checkboxes are connected to the master volume. The support told me that it is a known bug.
Maybe this helps.
Cheers
Frank
here you go..
Don't find a mistake. The only difference to me is that I do not have the monitor out link.
I hope franks advice will work.
If not - than my advice would be - contact the support..
Hi PdeCaumette. wait before calling/contacting support. check this out:
https://www.kemper-amps.com/fo…?postID=295974#post295974
it is a different out: the one on the thread mentioned was the direct one. but it was solved. cannot hurt to try that solution...
thanks Frank, but unfortunately, it didn't help...
it's a drag because I have had this unit for 3 years now, and never used the monitor out until now.
I hope that Kemper will take care of the defective KPA
I never used it live so this is not due to rough treatment...
Shit, open a support ticket and send them a backup file when you get the answer from your ticket via mail. The support is great, i think one of the guys will load up your backup file and analyse whats wrong.
They are very fast with their help.
Cheers
Frank
YES!!!
This did the trick!
Thank you so much man!
And thanks to everyone for being so supportive and willing to help: really a great forum!
Hi PdeCaumette. wait before calling/contacting support. check this out:
SOLVED - return input not working for reamp no matter what
it is a different out: the one on the thread mentioned was the direct one. but it was solved. cannot hurt to try that solution...
Great Michael 
YES!!!
This did the trick!
Thank you so much man!
And thanks to everyone for being so supportive and willing to help: really a great forum!
glad you could sort it out....it drove me crazy that time even because I had to use that output for reamping and there was no way I could activate it. it was support who mentioned the system reset which by the way I never did in my kemper owner life becaus I thought it was useless.
for this things it does the magic...
and yes, this is a great family of people which wants everybody happy with this machine...thus everybody helps each other. Imagine a whole world of brains activating for the others. Hard to believe a soluton is not found for every problem, except of course the hardware ones.
have fun and enjoy the ouptput
