You're flying circuits and you just turned downwind, ready to give your downwind call, but you can't get a word in as the radio (tower) is really busy. Before you know it, you're ready to turn final and still can't get talking to ATC.
Do you continue with your approach? Potentially without permission to land/Touch and Go? Or do you abort and go around? At which point should you have made the decision to go around due to no contact with ATC?
Aviate, navigate, communicate, that's the order of priority. Keep flying the plane and get a position call in when you can. Tower will have you in sight if you're in the circuit so will be well aware what you're doing, and if they haven't issued instructions to orbit or hold on the downwind I would continue to base can call as soon as I can. Often the radio gets very busy, just get the confidence to jump in with a short position call when you can. If on finals no clearance is coming, then it's a go around.
Another one being total radio failure... You're coming back to your airfield and are about to request rejoin when you realise comms are dead. Obviously there are the no radio procedures, flying triangles etc. But my understanding (it even says in the textbooks) is that these sometimes (usually) don't work.
Often each airport has it's own specific policy which can be found in the AIP, for example I learnt at Bournemouth which is inside class D, I seem to remember it was 3 orbits at the boundary, then enter and proceed to land.
If in doubt, one low approach and go around followed by a circuit to land would be sensible. These days I have most tower frequencies in my mobile phone, so would try that, I've even heard of someone getting a text from the controller saying cleared to land when their radio had packed up.