Dragnix.Net

Would it be okay to start someone elses RP up. For example if it's got loads of people but the author abandons it.

No. Many authors usually have a set plan for their Rp, and you would screw it up by restarting it. You should get their permission from that person first, and if enough people want it back, he or she should effectivly let you, or bring it back themselves.