Ruger P90DC Problems

Hey guys, just took my Ruger p90 out, brought it home took it out of the case to give it a good cleaning.. and i noticed the empty magazine had fallen out, so i stuck it back in sure enough the magazine release switch had completly fallen out so there was nothing to keep the mag in place.. I looked at it and cant figure out how or why it came out.. Anyways, one thing that concerns me is i was going to take it to the local gun shop to have them take a gander at it, but i'm not sure how they would take to it, I bought the pistol via private sale, as i'm only 19 ( law states that if i'm over 18 but under 21 i can purchase a pistol via pvt sale, for anyone who questioned that).. Any suggestions would be greatly apperciated.