I installed joomsef 4 succesfully but for some reason my jomSocial extension stopped working.

Are there any known issues that I'm not aware of?

How do I go about trouble shooting this problem?

Hi,
What exactly do you mean by "stopped working"? Do you see any error message? Is the whole JomSocial component not accessible or only some of its functionality? In the JoomSEF -> Manage Extensions page what is the Active handler for JomSocial?

when i said "stopped working" I meant to say i get "page not found".

Do i need to buy ext_joomsef4_community.zip
for extra $10 in addition to what I have already paid for JoomSEF 4 ???

JoomSEF on its own supports only the standard components present in Joomla. If you want correct SEF URLs to be generated for other 3rd party components, you need to install extensions for JoomSEF to support them. So if you want SEF URLs to be generated for JomSocial, you need to purchase the JomSocial extension for JoomSEF.
If you don't want SEF URLs to be generated by JoomSEF, you can also change the Handling method for JomSocial in Manage Extensions page in JoomSEF.