It should be fixable if multiboot is working. Try powering up your N900 with open keyboard to enter multiboot menu. In the menu press 0 (zero) or B to emergency boot Maemo. If it does not work try to post back a detailed description what happens. Also if you won't be able to boot Maemo could you try pressing S in multiboot menu and check if it gives you access to shell?

If all fails there is still hope. You can always boot Meego rescue kernel with rescue initrd and try to manually fix your Maemo installation. This can be tricky but there are plenty knowlegdeable guys on #nitdroid-help and #nitdroid freenode IRC channels to ask for further help.

Edit: link to rescue initrd with instructions