On most of these Lycoming or Continental engines the dipstick is combined with the oil filler cap. If, when you unscrew the oil filler cap it doesn't withdraw a long dipstick then it has broken off from the cap and fallen down the filler tube out of reach. There is usually a baffle to prevent it going right into the oil sump, but the aircraft is u/s until you get it out and fit a new filler cap / dipstick.
I should ignore the clever dicks and their less-than-helpful replies. In my opinion, if you don't know the answer you are only going to find it by looking it up or by asking someone who knows. Best plan is to get someone to show you on the aircraft. They can then also show you how tight to do up the oil filler. Many are damaged by overtightening and on the older spring-clip type, if you don't ensure all the clips engage, then the filler can come off in flight allowing oil to escape and the oil pressure to fall. One of our group members had this happen to him. Fortunately he spotted it in time, landed, and saved the engine (and himself).