Sorry to hear that.
The suggested fix using fastboot will probably work only on devices that have already been unlocked. Unfortunately, I don't know a reliable method. If access via adb is possible, I would try to restore the original files. Otherwise, there may be a way to install the original firmware again (from the SD card, for example).
Which firmware version is on the device? How did you rename the odex file (the file name, the file extension or both)?
It was on 10.6.1.14.10.
I renamed only the file extension.
Meanwhile I managed to recover it using this guide (maybe this is helpful for others):
First, my device failed to get the OTA Update because I deleted some system apps (see my original post). Then I found this post where another user restored original APKs in /system/apps and manage...