quick reply .. may miss a lot of important information, but you won't lose any data:

first, why you have the problem: likely you no longer had the kernel modules belonging to the stock kernel, which is basically what 'flashing' u-boot flashes. So your N900 is not booting as by default, u-boot will boot stock (attached) kernel until you instruct it otherwise

so, simplest way is to use flasher tool to load the kernel for kp53. You can find zImage on the repos.

Then once you're in Maemo, you can fix your u-boot properly