There can be a few variables with everyone. But key take aways:
Make sure you are healthy before you begin training again. No fever or symptoms. If a more serious illness consult with a physician if needed. We are not doctors and neither is the internet.
First day of riding should be easy zone 1/2 for 30 - 60 minutes max. See how you feel and see how your body responds the next day. If good the next day can introduce more zone 2 or even some lighter efforts.
From here depends on the type of training you are doing, how many days off you have had due to illness, type of illness and etc. If you get healthy enough to ride easy tomorrow and can introduce some freestyle intensity on the weekend you can resume where you were before your Illness on Monday. But I really like athletes to have 3 - 4 days of riding with a day or two of some type of effort before continuing where they were.