The first option should be to perform a UDL test. The following blog post has information on the same:

http://glennberrysqlperformance.spaces.live.com/blog/cns!45041418ECCAA960!939.entry

If this fails, you can then use the steps mentioned in the blog post by the SQL Dev team to troubleshoot your connectivity issue:

http://blogs.msdn.com/sql_protocols/archive/2008/04/30/steps-to-troubleshoot-connectivity-issues.aspx