what can you do if you find out the IMEI of a phone?what can there be done? and how?
i want to know what could happen if someone knows my IMEI

Someone could use that IMEI and write it in a blocked phone to circumvent the blocking of providers (stolen phones for example). This is not possible for all phones though and needs paid software.
Anyway, talking about such procedures is not welcome here so please don't ask for any details. Don't know if some moderator may think that I've already gotten too far - if so, you know what to do

That is why we request that you keep your IMEI numbers hidden. If you ever need to know it pressing *#06# while on the homescreen will get it for you. Only do this if your operator needs to know it for warranty purposes.