Semi-Dedicated is like a VPS server where people have complete access to it. If a semi-dedicated server is directly targeted, the main server has nothing to do with it.

@JoomlaZ: If a server is attacked, mostly the server load dont' rise. If the server load is rising, there could be number of factors, like,
high traffic and your server configuration not able to handle it,
poorly coded application/plugin activated,
bots crawling the websites etc

If possible provide some more details, output of top, ps and other commands OR have someone look into the server.