Try enabling device *without* assembling it's cover - try to press small black button (that is pressed by slider, normally). you will determine, if button itself is non-working, or slider mechanism malfunctioning.

Generally, use common sense. try to filter out possible sources of problem, for example, by checking them one by one and eliminating. Use Your sense (in tandem with common sense ) - try to re-create working situation without cover on it, and see, feel, etc what might be non working (in case, that previous, easier checks failed).

If You just don't feel able to do it, better pay someone who can. I'm not discouraging You, but checking why slider is malfunctioning is one of easiest hardware things on N900 I can imagine. After all, our talents varies, and it's better to pay someone, instead of breaking it even more (which is possible, if you can't determine the cause easily).

Either way, good luck, and write here about Your findings!

/Estel