HELP what have I done to my phone?

I was attempting to make a phone call on my S4 - it kept saying 'dialing...' and not going through. I was attempting to hit end-call or back or something else to get out of this call, and I saw a message that said something like "MMDI code running" - it wasn't exactly that but something like that - then I got back to my main screen but had no network connectivity. I restarted the phone, and instead of the little cosmic Samsung swirly thing on the black background that you get during startup, it was a black screen that said "SAMSUNG Custom" and an unlock icon, which I then touched to get to the unlock screen. I have no idea what I did, I have been running stock Android on this phone since the beginning. I don't want custom anything. What did I do? Can I undo it?

ETA I shut down again so I could go into recovery mode and wipe the cache - I can't get into recovery mode anymore but it did finally boot with the ordinary visuals. Are there some settings I should look at, to see if I've done something to the phone? I don't know why I can't get into recovery mode now.

Unless someone responds that has experienced this I would say this is a good question for XDA Developers.

I'm not registered over there - and I looked up the subject and everyone over there who came forward with a solution presented one that only worked for people who were rooted, even though half the people with the problem were not rooted.

Then ROOT the thing!! Xda is not known for bs answers i assure you! If you cant root it I can help ya. :cool: