Same thing happened to my phone with Straight Talk; Galaxy Proclaim. The problem occurred playing Fishing Diary downloaded from google play which may or may not be related. It was the only app that started making jerky movements (which I took advantage of to catch fish) whilst the others remained fluid.
SEQUENCE OF EVENTS
First the phone switched off midplay Fish Diary. Then it was bootlooping. Removed battery, sim card, still bootlooping. Then I went into Task Manager and forced apps closed having no idea what a couple were but the names were benign. Then "Mobile Network Not Available"
Everything in settings>status> Phone number: unknown, PRL version: not available or Mobile network state: disconnected, etc...
As a matter of fact, this problem is all over the ST forum but moderators are asking everyone to move conversation to email or private so no solutions there!
Sim card: Fine
Removal of sim card and battery. Still had problem
Factory reset: Nada! Not bootlooping but no signal.
Someone suggested Samsung mobile assistance but they don't care. ST support follows a dummy script and told me to mail in the phone. The whole process will take a minimum of 14 days without a phone and I still like it is fixable and not hardware related at all!
I've been rummaging in the Howard forums but no one has addressed the specificity of this problem yet.