To reset the GuruPlug, there is a good tutorial at plugcomputer.org: Reflashing images on the GuruPlug
If you did not destroy the whole u-boot (if you plug your GuruPlug in, you can see U-Boot starting and it at least runs until it shows "Hit any key to stop autoboot:"), you can start the tutorial from "Burning Kernel and Rootfs".
There were a few things that I struggled with. Therefore, this tutorial should be considered as extension to the tutorial on plugcomputer.org. Especially setting up tftp and getting the right files was tricky. I hope the screenshots also help to verify that you are on the right track.
Things which are needed are:
All the commands mentioned in the tutorial and here are executed on the Marvell-prompt. Right after u-boot started, press any key to get into the Marvell-prompt. Minicom will show something like this:
Configure the network and burn the kernel image
Burn the root file system (rootfs) and set environment variables
Getting the rootfs per tftp is not visible in the picture. The command is:
>> tftp 0x6400000 rootfs.ubi.img
It will take quite a while to download. Also erasing and writing the NAND will take some time. Since writing the NAND does not show the progress, just wait until the OK-message shows up.
to reset the GuruPlug. Go into the Marvell prompt again if you want to update U-Boot. Otherwise, just let it run and watch the boot process.
This step is optional, but it should not bother if you do it.
to reset the GuruPlug. Just let it run (do not go into the Marvell prompt) and watch the boot process.
Here is good information on how to get the most recent DENX U-Boot version