Guild Raids - cannot join

So a newbie here, pls forgive if this has been asked many times.... Have read what I could find but am still lost.

I was in a guild and could do Raids, then all of a sudden I could not enter raids anymore. From what I understand the leader has to start a raid for me to enter. Maybe my guild did not start some.... So I joined a couple of other guilds and even though I saw in their chat that a raid was started, I could not take part.... I've purchased the lightspeed pack so I know I have characters that meet the minimal requirements.

I love raids, and wanna do more! What's the best way to find them? Is there a time limit when I join a guild before I can partake? Is there a way to see why I cannot join?

I have spoken.... hahahaahaahha