Open 80% of the keyboard slide out. Just enough to see "U" then press "U" and plug in MicroUSB.

If it doesn't work then it's either your MicroSD is broken or you've done it all wrong. Never once this would failed on anyone here. 100% all the time guaranteed that it will flash. N900 is for saint users Normal users will find it glitchy and buggy and even what happened to you. I never had any problem with reflashing mine.