Logitech don't send out Z-5500 parts it seems anymore, they replace the whole set for the new Z906's, but you have to be in their 2 year warranty. That's what they did when mine went kaput and kept blowing the slow blow fuse, I had to fix my Z-5500's myself after that... Turned out to be a bad capacitor on the PSU in the subwoofer

. So working Z-5500's again and gave my replacement Z906's to my brother.
Also try the Firmware Version mode below it will also do a full factory reset when you do that.
If you are using the Z-5500's on the Optical or coax digital mate try changing the digital mode on your sound card a few times, it may stop the system going into standby I have seen mine do that when the sound card is sending out rubbish threw the digital and sometimes even locks up the whole pod. Also try removing the digital cables and using the analogue cables as a test, see if it still does it, if not it's 100% the sound card sending out rubbish via the digital or the pods digital inputs are fried (which I doubt). Common fault is the sound cards sending out rubbish to the pod and crashing it or it goes into standby. What sound card are you using ? If it has a dolby or dts mode, change the mode a few times and see if that helps too. I have had Dolby Digital Live on my X-Fi Titanium-Fatal1ty Pro cause lock-ups and standby mode, fix was to change to normal mode then back to Dolby Digital Live, it's not the speakers at fault but the sound cards sending rubbish data. Diagnostic Mode on the Z-5500's may show this too (read below how to enter Diagnostic Mode).
2nd Skin: Put the control centre in Standby (eg, subwoofer power switch on, but control centre in standby) then press and hold the following buttons for about 6 seconds: Input, Effect, Settings, and Mute buttons simultaneously. The display should now read "2nd Skin On". Your power button will now be (indigo) or (violet) from the default "red" and the sound bars will also change, although you can change it back to default by doing the same procedure.
Boost Mode: Turn the volume knob up as high as possible, then keep turning. It will now switch to boost mode and get even louder. (Unless you want to kill yourself, I don't suggest this.) Boost up to +11
Up to Boost 22: Hold down the Mute button and turn the volume up, this can be combined with Boost Mode for some serious damage. (Again, Unless you want to kill yourself, I don't suggest this.)
Diagnostic Mode: Put the device into standby, as above, then press and hold Input and Effect buttons simultaneously for 6 seconds. The control centre will now show more information about the digital feed it gets from the Coax or Toslink connection for 5 secs (only for digital connections not the analogue ones).
Firmware Version: Use this feature to view your firmware version. (I have v2.1.0 BTW) While on standby mode again, press and hold the following buttons, Input, Effect, and the Settings button for about 2 seconds then press the power button, remove your fingers off all the 4 buttons (including power button), then press the power button again.