It sounds as if you have already rebooted your Tablet (turned it off and on again), which is always the first thing to try when it is not working.

The next thing to try is to remove the battery for half a minute. Turn off the Tablet first, since you are able to to do that. (Sometimes a Tablet will be frozen so badly that you can't turn it off, in which case you have to pull out the battery while it's on, but it's better to turn it off first if you can.)

If it still won't work after you replace the battery, the third thing to try is reinstalling the firmware (also called reflashing or just flashing). If you haven't done that before, you will need to read the wiki at http://wiki.maemo.org/Updating_the_tablet_firmware and you will have to do some downloads to your PC.

Beyond that, you might have to get your friend to pay for a replacement.

EDIT 02-22-10 : I forgot to mention data corruption. This can happen any time, typically during file transfer, and typically leaving a corrupted file on one of the media cards. It can have all kinds of weird effects, although most often the Tablet goes into a "reboot loop" - it keeps turning itself on and off.

To find out whether your problem is caused by a corrupted file on a media card, remove both cards and see whether your Tablet is now behaving as it should. If that seems to fix the problem, you then have to figure out which card is corrupted and how you are going to save the uncorrupted files on it. Sometimes you can't delete the corrupted file except by reformatting the whole card, and that wipes out all the data on it.