There are two main types of diabetes:
Type 1 diabetes means that the pancreas has stopped producing insulin. You have to relace this with injected insulin. This is vital to keep you alive and you have to take it every day.
Someone with Type 2 diabetes still produces insulin, but it either is not enough or cannot get to the cells to do its work. This type of diabetes is often treated with tablets.
If you have Type 1 diabetes, you may be very poorly very suddenly until you are treated.
But Type 2 diabetes develops much more gradually so it may not come out of the blue.