Whatever the reason for time off coming back can be tricky, not knowing where to start and not accepting some loss of fitness can lead to over-training (which might be why you had time off in the first place).
Two weeks to a month is unlikely to make much difference but after this you will not be able to jump right back in to training as you were.
Start off with some very short easy workouts (30 mins) at an easy speed and see how you react.
Even heart rate zones can change especially after some serious over-training or some illnesses such as flu, covid or even a heavy cold. You don’t want to start off with a test so just do workouts to how you feel, keeping an eye on the data to avoid over doing it, until you have built up some fitness.
Finding a baseline to build from can be tricky but if you start off with short easy workouts three or four times a week you can look to start building some longer rides once per week. You want to be coming off the bike feeling ok and like you could do more if you chose, not struggling to unclip.
Building endurance with easy zones 1-2 rides is usually a good place to start. It does not produce too much stress and can be a good way to relax and get some positive feelings from exercise. Adding ten to fifteen minutes to one ride week on week with an easy week every third or fourth week works well.
Recovery is as important as getting out on the bike, so plan recovery time and take it whether you feel you need it or not. The costs of more recovery than you need are far less than the cost of over-training.
Frequent training provides greater fitness gains than long intermittent training. Small regular amounts of training will help you make gains rather than totally battering yourself and needing another long break where you will be right back where you started.
Higher intensity training like interval sessions, hill climbs, sweetspot or temp work can be a good way to build fitness once you have spent some weeks building endurance. It’s often better to start with something just above endurance (tempo/zone 3) and build from there leaving zone 5 (VO2max/above threshold) training until you have built some good tempo and threshold (zone 4) fitness.
Patience is key, you will be keen to get back to it but you cannot force fitness to happen more quickly. You may find you have some setbacks, especially when you start to introduce some intensity, just go back to your baseline and listen to your body.
Talking to an objective person who understands cycling training can help.