[7" Kindle Fire HD Q&A, Help & Troubleshooting] pjeby:
Yeah, that was a bad idea. ;) Better to use "/system/app mover" app for that stuff, just install normally and then move it. I only put stuff in system/app by hand if somebody else has said it's the only way to do something. And then I triple-check permissions. If by "putting" you mean you just copied it there w/out changing permissions, that's probably your problem. I would just delete it and not do that again, but w/out knowing exactly what you did, and factoring in the part where I've only been doing this since yesterday... well, good luck. ;)

Use KFFirstAide. Uninstall every driver related to Kindle or other android device you can find. If necessary, hook up the Kindle to do it.

Use KFFirstAide to install the Amazon drivers first - I think it's option 12. Try options 15 and 16 (the two fastboot methods) to try and get into your device. 15 only works if you have adb working, 16 can work without it. In all cases make sure that you follow what KFFirstAide says to do exactly, i.e. what to have on, off, connected, disconnected, etc. It's not like outsourced tech support telling you BS about rebooting your computer to fix your modem. ;)

Anyway, fastboot is a success if you get to a screen with a logo on it. Nothing else is going to happen, but the KFFA diagnostics should tell you "yep, it's in fastboot".

In my case, I then recovered by getting into the adb shell after this, by telling KFFirstAid to reboot it normally. And from there I fixed my goofs in adb. (But this *only* worked after I'd gone through many loops of making sure I got ALL the drivers out.

In general, when I had devices not starting, I could usually fix by uninstalling drivers again (be sure to go to add/remove programs and search thoroughly, refresh the screen w/F5 if you've left it open between removing things). Between yesterday and today I reinstalled those things a dozen times in order to get back in. The problem I think is that at some point if the device doesn't boot up properly, it stops in a mode where Windows thinks it's a different device.

Oh yeah, to get into adb, I sometimes had to update the driver from "ADB device" to "ADB composite Device", by telling Windows to hunt in the KFFirstAide directory for a driver. Sometimes you have to wait for the Kindle to fully boot up (and almost a minute later) before you get to adb, but when I did "boot normal" from fastboot mode (using KFFirstAide), I could get into adb only *before* the boot-up finished; after that it'd disconnect and it'd be too late to use the shell to undo my screwups.

I found out most of the above by searching the forums here, and occasionally by trial and error.

My advice: don't brick it in the first place if you can avoid it. Getting things back is a royal pain. ;)

(Today, after all the junk I tried, I had apparently messed up my Google Play, so I spent many more reboots getting that working properly again, only to lose all my desktop icons and widgets from my launcher. All of that is more-or-less fixed, but I'm exhausted and don't want to have to touch anything else ever again. If I'd known it would all be this much trouble I probably wouldn't have bothered rooting, or at least not trying to fix the wallpaper. I now know far more about how this stuff works than I ever wanted to.)

What widgets, and what do you mean by "load"?

I found that I couldn't add any new widgets to certain launchers without installing AppWidgetPicker.apk, and I had some challenges during my lengthy reinstalls of Play Store, but that was it. I honestly am not sure when that broke; it might've been during my initial rooting. But it's been so long since I'd added a widget anywhere that I wouldn't know if 7.4.1 broke it or me rooting 7.4.1 broke it.