You won't loose any data if you just reflash ur OS a.k.a. rootfs another a.k.a. firmware also known as FW and referred to fiasco you will need windows preferably XP:
A)
1. Go to http://tablets-dev.nokia.com/nokia_N900
2. There are clear instructions on how to find and enter your imei
3. Throw the EULA in a trash can and click I agree
4. Download RX-51_2009SE_20.2010.36-2_PR_COMBINED_003_ARM (this or the one specifying your region)
5. Download Maemo flasher 3.5
6. Install Maemo flasher
7. Copy RX-51_2009SE_20.2010.36-2_PR_COMBINED_003_ARM (this or the one specifying your region) to Maemo flasher's directory (e.g. C:/program files/maemo)
B)
1. Open maemo flasher from start ----> all programs ----> maemo
2. Type: flasher-3.5 -F <FIASCO image> -f (RX-51_2009SE_20.2010.36-2_PR_COMBINED_003_ARM in this case)
3. Hit enter then plug in your usb cable to the computer then while off open ur N900's keyboard and press u and keep holding it
4. after some seconds a cmd window will pop out showing flashing status probably for 5 mins
$. After it finishes close everything and disconnect usb
p.s. why don't they just provide a one name instead of several a.k.a's