Both are acceptable, but the first is more commonly used, at least in American English. The first uses the present continuous and the second uses "panicked" as an adjective.

Maybe you should note that "to panic" does mean to be afraid, but has different connotation that to say "to be afraid" or "to be fearing". Panic implies (to me, at least) that a a person is unable to really function in an effective way. (And therefore, often to be able to react intelligently and appropriately their situation.) If you were to read manuals or books about survival or first aid, you might very well come across the phrase, "don't panic".