If you define that your coin is a "fair coin", there is always a 50/50 chance of its landing on head or tail on each toss, no matter when. There is 1/2^5 (=0.03125) chance that you get heads 5 times in a row, and that's what you did get in the tossing you mentioned.
The problem is how you know it's a fair coin. Theoetetically, there is no way to know it is. You must toss it infinite times to be 100% certain that it is fair or not. Practically, you can only get close to this ideal knowledge,, by tossing it as many times as you can, say 1,000 times, and it came out 50/50. (Yet, you can never be 100% sure that it is fair if the the number of tossings is finite.).
For an unknown coin hat comes out heads five time in a row but without a defined (or assigned, to be more correct) tossing probability, it may be fair coin, a 70% head coin, a 90% tail coin, etc. So, the probability of tossing is undefined for this unknown coin. Until you define ( assign, to be more correct) the coin's probability of tossing, the probability of tossing of that coin remains forever undefined.