Mulitplayer Offline

When I open Minecraft it says "Play Offline" other than "Play". I have checked my wireless and everything so the error is related to minecraft. Sometimes it says "play" so I go to my server along with others and I can see the amount of people online then when I click to play it gives me a super long URL that doenst work, with error 505 or something like that. PLEASE HELP I have a huge server thing today!